Legacy data access: What you need to know post S/4HANA go-live

Author: Priyasha Purkayastha, Global Content Manager, TJC Group | Co-author: TJC Americas team, TJC Group

The go-live of your S/4HANA system is a milestone worth celebrating, but it is far from the end of your transformation journey. One of the most overlooked aspects of any S/4HANA implementation is planning how to access legacy ERP data after the transition. According to recent findings, 83% of organisations underestimate legacy data access requirements until it is too late. This article explores the critical use cases for data access after S/4HANA go-live and how a modern historical data hub can help your organisation stay compliant, cost-efficient, and operationally resilient.

Introduction

Migrating to SAP S/4HANA is one of the most significant transformations an organisation can undertake. The new platform delivers real-time analytics, in-memory computing, and a simplified data model, representing a substantial leap forward from SAP ECC. However, once the excitement of go-live fades, a critical question remains: what happens to the data in your old systems?

Legacy ERP systems contain decades of business-critical information, from financial records and tax archives to HR data and customer transaction histories. This data does not simply lose its value the moment a new system goes live. In fact, legal requirements, business continuity needs, and operational demands often make access to historical data essential for years, sometimes decades, after migration.

Despite this reality, legacy system decommissioning is frequently the last item on the project plan. Many organisations either leave their old systems running at great expense or freeze them on virtual machines, neither of which is a sustainable or compliant long-term strategy. This article draws on insights from TJC Group’s webinar on post S/4HANA go-live legacy data access use cases to help you understand the challenges and solutions available.

Why legacy data access matters after S/4HANA go-live

Regardless of whether your organisation has chosen a greenfield, brownfield, or Selective Data Transition (SDT) approach, the need to access legacy data persists. Research shows that only around 10% of the data organisations hold in their systems is actively used. The remaining 90% is legacy information, some of which is redundant, but a significant portion is required for compliance and business purposes.

A common misconception is that a brownfield migration eliminates the need for legacy system decommissioning. This is not the case. A technical upgrade does not prove that data is complete or unchanged, meaning the S/4HANA system cannot function as a tax archive for data originally created in the legacy system. Auditors may request to see the origin of legacy data in its original form, which requires a dedicated solution.

Furthermore, with 59% of companies now fully or partially live on S/4HANA and SAP ending mainstream ECC support in 2027, the urgency to address legacy data access has never been greater. Organisations that fail to plan for this will face escalating costs, compliance gaps, and operational disruptions.

Watch our webinar on legacy data access use cases post S/4HANA go-live for more information:

The hidden costs of keeping legacy systems alive

Many organisations assume that simply keeping their legacy SAP ECC or R/3 systems running is the easiest path forward. However, the hidden costs of maintaining legacy systems are far more significant than most realise.

On average, 60 to 80% of IT budgets are allocated to keeping legacy systems operational. These costs include SAP licensing fees, infrastructure maintenance, database support, and the specialist personnel needed to manage outdated technology. As these systems age, finding experts who can maintain them becomes increasingly difficult and expensive.

Beyond the obvious financial burden, the true cost of keeping legacy systems and data extends to accumulating technical debt, overloading IT teams with outdated technology, incurring expenses for redundant software licences and security measures, and dealing with the environmental impact of running and powering old applications. In a RISE with SAP scenario, memory costs grow geometrically rather than linearly, making it even more critical to manage what data resides in your HANA database and what can be stored elsewhere.

Key use cases for legacy data access post go-live

The webinar highlighted several critical scenarios where organisations need to access legacy ERP data after their S/4HANA go-live. Understanding these use cases is essential for building a robust decommissioning strategy.

Tax and audit compliance

Tax and audit compliance is arguably the most important use case for legacy data access. Organisations are legally required to retain financial records for extended periods, typically seven years in the United States, and up to ten years in Canada and across Europe. Auditors may request access to historical transaction data, general ledger entries, accounts payable and receivable records, and tax-relevant documents at any time.

Critically, data must be kept as a tax archive, extracted and stored as a flat file, to prove it has not been modified. A decommissioning solution that does not provide proof of completeness and proof of non-modification will not meet tax and audit requirements. This is why ETL (Extract, Transform, and Load) solutions based on database-to-database transfers often fall short, as they may not guarantee data integrity. The importance of SAP data archiving when migrating to S/4HANA cannot be overstated in this context.

Mergers and acquisitions

Mergers and acquisitions (M&A) create particularly complex legacy data challenges. When two organisations merge, they often bring together multiple ERP systems, each containing years of historical data. This data must remain accessible for due diligence, financial reconciliation, and regulatory reporting.

In M&A scenarios, the acquiring company may need to access the target’s legacy SAP or non-SAP data for integration planning, historical financial analysis, and compliance verification. A centralised historical data hub becomes invaluable here, allowing both organisations’ legacy data to be accessed from a single platform without maintaining multiple costly legacy systems. Understanding how to decommission both SAP and non-SAP legacy systems is critical in these situations.

Divestitures and carve-outs

Divestitures present the opposite challenge to mergers. When a business unit is sold or spun off, the divesting company must separate the relevant data and ensure that both parties retain access to the historical records they need. This often involves extracting data for specific company codes, fiscal years, or business units from the legacy system.

A real-world example from TJC Group’s experience illustrates this well. A global manufacturing company undertook a brownfield S/4HANA implementation of financial data that deliberately excluded divested companies. The legacy ECC system contained data for these divested entities that still needed to be preserved and accessible. Without a proper decommissioning solution, the company would have been forced to maintain the entire legacy system solely to access this subset of data.

HCM transitions and HR data retention

Human Capital Management (HCM) transitions represent one of the longest-tail use cases for legacy data access. Many organisations migrating to S/4HANA are simultaneously moving their HR functions from SAP HCM to SAP SuccessFactors or other cloud-based HR platforms. However, the historical HR data in the legacy system must be retained for remarkably long periods.

HR data, including pension records, payroll histories, and employment contracts, must be held for over thirty years in specific countries and industries. This makes it impractical to keep a legacy system running just for HR data access. A dedicated legacy system application provides a far more cost-effective and compliant solution, ensuring that HR records remain accessible long after the source system has been retired. For a broader understanding of data retention obligations, explore our guide on data privacy, its importance, regulations, and more.

Warranty claims and product liability

Manufacturing and consumer goods companies frequently need to access legacy data for warranty claims and product liability investigations. These enquiries can arise years or even decades, requiring access to production records, quality data, and supplier information stored in the legacy ERP system.

It is important not to underestimate the user’s requirement to access this historical data, whether for internal investigations, external audits, or legal proceedings. A modern data access solution ensures that this information remains readily available without the overhead of maintaining the original system.

The role of a modern historical data hub

TJC Group’s Enterprise Legacy System Application (ELSA) addresses these use cases by providing a modern historical data hub for accessing decommissioned system data. Built on the SAP Business Technology Platform (BTP), ELSA makes it possible to access legacy data, documents, and reports either from the cloud application itself or from SAP S/4HANA through a UI5 application.

ELSA serves as a centralised platform where data from multiple decommissioned systems, both SAP and non-SAP, can be stored and accessed through a single interface. This is particularly valuable for organisations with complex system landscapes resulting from years of growth, mergers, and acquisitions. The journey from sunsetting to decommissioning legacy systems becomes far simpler with a purpose-built application.

Key capabilities of ELSA include dynamic and static reporting on historical data, proof of completeness and non-modification for tax compliance, data privacy features for GDPR and other regulatory requirements, a straightforward interface accessible to users unfamiliar with SAP GUI, and the ability to generate new views without any custom development. A case study involving a global US manufacturing company demonstrated that decommissioning with ELSA saved over $250,000 per year in ongoing maintenance and storage costs.

Security and compliance risks of legacy systems

Keeping legacy systems alive after S/4HANA go-live introduces significant security and compliance risks that grow more dangerous over time. Legacy systems that are no longer actively maintained become prime targets for cyberattacks, as they do not receive security patches and updates.

The strategic imperative of decommissioning legacy systems for better cybersecurity is clear. Unpatched legacy systems expand an enterprise’s attack surface, creating vulnerabilities that modern security frameworks struggle to manage. A successful ransomware attack on a legacy system could result in costs far exceeding the expense of a proper decommissioning programme.

From a compliance perspective, SAP Information Lifecycle Management (ILM) provides tools for defining data retention policies and managing data destruction at the end of retention periods. However, legacy systems were not originally designed to handle modern data privacy requirements such as GDPR. Organisations remain liable for data privacy compliance in these older systems, making decommissioning with a compliant solution not just advisable but essential.

Additionally, for organisations using RISE with SAP, the legacy system will not be accessible after a certain timeframe, usually 12 months, as SAP includes the buy-back of SAP ECC perpetual licences. This makes decommissioning the legacy system an absolute necessity rather than an optional consideration.

Best practices for planning legacy data access

Drawing on TJC Group’s 25 years of experience in data volume management, here are the essential best practices for planning legacy data access after S/4HANA go-live:

Start planning early

Legacy data access should be part of your S/4HANA migration strategy from day one, not an afterthought. Begin planning 12 to 18 months before your target go-live date to allow sufficient time for data analysis, extraction, and validation. A comprehensive guide to navigating legacy system decommissioning can help you structure this process.

Conduct a thorough data inventory

Identify all data, documents, and reports that need to be preserved from the legacy system. Categorise data by type (financial, HR, operational), retention requirements, and access frequency. This inventory forms the foundation of your decommissioning strategy.

Ensure tax archive compliance. Any decommissioning solution must provide proof of completeness and proof of non-modification to qualify as a valid tax archive. Solutions that rely on database-to-database transfers may not meet these requirements.

Address data privacy requirements

Ensure your legacy data access solution includes capabilities for data anonymisation and deletion in compliance with GDPR and other data privacy regulations. Legacy systems were not designed for these requirements, but your organisation remains liable.

Consider all use cases

Map out every scenario where legacy data might be needed, including tax audits, warranty claims, M&A activities, and HR record requests. This ensures your decommissioning scope covers all critical data. Understanding SAP data archiving processes will also help you determine what data should be archived versus decommissioned.

Control your HANA database growth

Keeping unnecessary data in the HANA database increases costs significantly. Implement a strategy to manage HANA database growth by ensuring only actively used data resides in memory, while historical data is stored cost-effectively in a legacy system application.

Anticipate user requirements

Business users will have new requirements after the legacy application is retired. Define the scope of extraction carefully and try to anticipate user needs during the project to limit costly custom developments later. Understanding the key differences between SAP S/4HANA and SAP ECC will help you identify where legacy data access gaps may arise.

Conclusion

Planning for legacy data access after S/4HANA go-live is not optional; it is a critical component of any successful transformation. The use cases are varied and significant, spanning tax and audit compliance, mergers and acquisitions, divestitures, HCM transitions, and warranty claims. Organisations that neglect this aspect of their migration risk escalating costs, compliance failures, and operational disruptions.

Partner with specialists. Legacy system decommissioning requires deep expertise in both SAP systems and regulatory compliance. TJC Group, with over 25 years of experience, helps organisations navigate this process with its SAP-certified ELSA solution. Contact us today to discover how we can help you take control of your legacy data landscape.

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