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Start with the audit context
Before setting controls or assembling evidence, confirm the jurisdiction, reporting framework, engagement type and criteria the auditor will use. Agree with the relevant audit team or assurance function what evidence it expects and how exceptions should be handled. Neither a checklist nor a modernization framework guarantees an audit outcome.
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For covered U.S. federal engagements, the Government Accountability Office’s Federal Information System Controls Audit Manual (FISCAM) provides a framework for assessing the design, implementation and operating effectiveness of information-system controls. GAO’s June 2026 revision applies to attestation engagements and performance audits beginning on or after 1 October 2026. It is not automatically binding on private-sector organizations or non-U.S. audits; check the applicable manual and standards for the engagement. GAO FISCAM
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Build an evidence-backed view of the estate
Prioritization and planning are difficult to defend if the organization cannot show what it operates, what depends on it, and what information it holds. Create or update an inventory that connects each legacy system to its owner, business or mission purpose, key dependencies, data assets, known risks and current status.
UK guidance recommends a complete, accurate and regularly updated information asset register. It describes recording the type of information, where it is stored, and its security and handling requirements, alongside thorough documentation of changes and additions. Managing legacy technology
- System and ownership: identify the service or system, accountable owner and operational purpose.
- Dependencies: record integrations, upstream and downstream systems, infrastructure and other relationships that could affect migration or continued operation.
- Information assets: note what data is held, where it is stored, and how it must be protected and handled.
- Condition and risk: capture known vulnerabilities, support constraints, control concerns and consequences of failure, with dates and accountable reviewers.
- Change history: preserve an updated record of additions, changes and decisions rather than relying on an undocumented current-state snapshot.
The register is a foundation for decisions, not a substitute for the detailed records needed to show how a particular modernization was designed and controlled.
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Prioritize using risk and mission impact
Document why one system is being modernized ahead of another. The UK Legacy IT Risk Assessment Framework describes a qualitative risk assessment and says systems rated red should be prioritized for immediate action. Its page notes that the framework is under review to align with the updated government definition of legacy IT, so check the current version before adopting its definitions or criteria. Legacy IT Risk Assessment Framework
Use criteria that make the decision explainable to people outside the delivery team. Depending on the organization and applicable criteria, these may include impact on mission or business services, security and operational risks, data sensitivity, supportability, dependency complexity and the consequences of continued operation or a failed migration. Record the assessment, rationale, owner, review date and any accepted residual risk. Do not treat a UK risk rating as a universal threshold where it is not applicable.
Make each system’s modernization plan reviewable
For U.S. federal legacy systems, GAO identifies three minimum elements for a documented modernization plan: milestones, a description of the work necessary, and details on disposition of the legacy system. Those elements are a useful planning baseline, but their formal applicability depends on the engagement and organization. GAO-25-107795 (2025)
For each system, translate that baseline into a plan that reviewers can follow from the current state to the intended outcome:
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- Milestones: identify dated or otherwise measurable decision and delivery points, responsible owners, and the evidence that will show each milestone is complete.
- Work: describe the work packages needed to modernize, including relevant design, build, migration, integration, validation and operational-readiness work.
- Dependencies and decision points: show what must happen first, who approves material choices, and how unresolved dependencies or changed assumptions will be escalated.
- Legacy disposition: state whether the legacy system will be retired, replaced, retained temporarily, or handled through another defined outcome; specify the conditions and evidence for that outcome.
- Risks and exceptions: link material risks, mitigations, approvals and accepted exceptions to accountable owners and review points.
The last two items make the plan more operationally useful; GAO’s stated minimum calls specifically for disposition details, but does not prescribe a universal template or evidence repository.
GAO’s 2025 review found that among 11 selected critical federal legacy systems, three had fully documented modernization plans, six had partially documented plans and two had no plans. These are findings about that selected federal sample, not estimates for all government systems or commercial organizations. GAO-25-107795 (2025)
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose a migration approach that can be controlled
Phased or iterative migration is one option, not a universal requirement. UK government guidance recommends continuous improvement and iterative or phased migration to help prevent future accumulation of legacy technology, while emphasizing compatibility, integration, documentation and control of the remaining legacy estate. Managing legacy technology
Compare delivery options against the system’s actual constraints rather than assuming one method is always safer:
- Business continuity: what service interruption or degraded operation could each approach create?
- Risk reduction: how soon does the approach reduce the most important operational or security exposure?
- Complexity and interoperability: what temporary coexistence, interfaces or integration work will be required?
- Cost and schedule exposure: where could rework, dependency delays or parallel operation extend effort?
- Validation: can the organization test data completeness, accuracy, access and relevant controls before relying on the new system?
- Disposition clarity: does the approach have a credible route to the documented outcome for the legacy system, rather than leaving an indefinite parallel estate?
The available official guidance does not establish a universal ranking of migration approaches. Record why the chosen path fits the system, what conditions would trigger a change, and how the remaining legacy environment will be managed while work proceeds.
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Maintain evidence as work changes
Audit readiness depends on records that reflect what actually happened, not just the approved plan. Keep the relevant evidence in the organization’s normal records systems and make it traceable to the system, milestone, decision or control it supports. No single repository or universal checklist is prescribed by the cited guidance.
- Retain approved plans, design and control changes, and the approvals associated with them.
- Record exceptions, risk acceptances, decisions and their rationale, including the responsible approver and review point.
- Keep validation outcomes and evidence of remediation or unresolved findings connected to the relevant migration stage.
- Update the asset register and dependency records when systems, data locations, integrations or handling arrangements change.
- Track residual risks and the operating status of systems that remain in service during or after migration.
- Preserve evidence that the legacy system reached its planned disposition, including any approved deviation from the original plan.
Set ownership and a cadence for keeping these records current. A stale inventory or a plan that no longer matches delivery weakens the link between the rationale for modernization and the outcome being presented for review.
Use guidance within its limits
GAO’s plan findings and FISCAM address U.S. federal contexts; the UK government material addresses UK public-sector risk and migration practices. These sources converge on documented planning, awareness of risk and controlled change, but they do not combine into a single standard for every organization. Apply the criteria that govern the actual audit, use other guidance as relevant practice, and have the accountable assurance or audit function confirm expectations as the engagement evolves.
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