Modernize around the services the business must keep running—not around the deployment date. Set measurable limits for disruption, map the dependencies that keep each critical service working, test recovery and data risks, and make clear who can pause the change and who owns any residual risk. The regulatory examples below are specific to UK financial services; the planning examples are from US federal modernization and are not rules for every business.
What does “without breaking it” mean?
A migration is not successful merely because new software has been deployed. It must also preserve the outcomes customers, markets, patients, citizens, or counterparties rely on, within disruption limits the organization can tolerate. That means planning for the service as a whole: the technology is only one part of the operation that delivers it.
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The UK Financial Conduct Authority (FCA) defines operational resilience as “the ability of firms, financial market infrastructures and the financial sector to prevent, adapt and respond to, and recover and learn from operational disruption.” FCA Operational resilience gives this service-focused approach a regulatory context for firms within the rules’ scope.
Before choosing a migration design, identify the jurisdiction, sector rules, important services, data obligations, dependencies, and target environment that apply to your organization. The FCA’s requirements are not a universal rulebook, and US federal oversight examples are not a proxy for every regulated business.
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What should a migration plan make visible?
A practical plan connects service outcomes to the work, the evidence needed to proceed, and an explicit end state for the legacy system. The following is a planning framework, not an official regulator-prescribed scoring model.
| Plan element | What to decide | What to make visible |
|---|---|---|
| Service outcomes | Which services must continue, and what disruption can each tolerate? | Named service owners, measurable impact limits, and the customer or business outcomes at risk. |
| Dependencies | What people, processes, technology, facilities, information, and third parties enable each service? | Maps showing the dependencies and the relationships that could affect continuity if one changes or fails. |
| Sequence and milestones | What work must happen first, and what must be demonstrated before the next stage? | Milestones, decision points, owners, and criteria to advance, pause, or remediate. |
| Risk and test evidence | What vulnerabilities, data risks, operational dependencies, and recovery limits need to be tested? | Test results, unresolved issues, remediation owners, and the conditions that trigger a controlled pause. |
| Accountability and oversight | Who owns service outcomes and residual risk, including where a provider performs the work? | Executive visibility into schedule, issues, action owners, decisions, and risks accepted or awaiting action. |
| Legacy disposition | Will each legacy component be retained, modified, replaced, or retired, and when? | A defined end state and the work, dependencies, and timing needed to reach it. |
| Recovery and learning | How will teams respond, recover, communicate, and act on lessons from tests or incidents? | Recovery responsibilities, communications arrangements, test findings, and tracked follow-up actions. |
The FCA expects firms within scope to understand and map the dependencies supporting important business services, including third parties. This is why a system inventory alone is not enough: a service can depend on staff procedures, facilities, information flows, or external providers as well as the platform being changed.
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How should work be sequenced and tested?
- Set the service boundary. Name the service being protected, its owner, the outcomes it delivers, and its measurable disruption limits. Confirm which organizational and regulatory obligations apply before using another jurisdiction’s example as a rule.
- Map the current operation. Document the people, processes, technology, facilities, information, and third parties involved in delivering the service. Mark dependencies that could fail together or make recovery harder.
- Define the future state and legacy end state. State what is changing, what remains, and what is to be retired. Make the disposition of the legacy system explicit rather than allowing it to persist by default.
- Break the work into milestones. For each stage, specify the work, evidence required, accountable owner, and decision-maker. Define what would justify proceeding, pausing, or remediating before the next step.
- Test realistic disruption and recovery. Use scenarios relevant to the service and its dependencies, including loss of a provider or technology component where appropriate. Check whether recovery can meet the service’s limits, and record failures, issues, and actions rather than treating a completed deployment as proof of resilience.
- Control the decision to proceed. Give executives a view of progress, schedule changes, unresolved issues, action owners, and decisions. Ensure the people accountable for the service can act on that information.
- Learn and close the loop. Track test and incident lessons through to remediation, update dependency and recovery plans, and verify that the planned legacy disposition has actually been reached.
These steps do not prescribe a particular cloud pattern, cutover design, or replication strategy. The right technical approach depends on service criticality, data obligations, architecture, dependencies, and the organization’s ability to test and recover. Compare possible approaches by their continuity exposure, dependency and third-party risk, testability, recovery, data and audit evidence, reversibility, governance clarity, and explicit legacy retirement.
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UK financial services: FCA operational resilience
The FCA’s operational-resilience rules came into force on 31 March 2022. For firms within the defined scope, the transition period ended on 31 March 2025: firms were expected to complete mapping and testing to remain within impact tolerances for each important business service and make necessary investments. The FCA page, updated 15 September 2026, describes those expectations; the deadline is not an upcoming one.
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Third-party delivery does not transfer a UK firm’s accountability for its regulatory responsibilities. The FCA regards cloud services used to deliver important business functions as potentially material outsourcing, but requirements vary by firm type and function. Do not assume that all providers or arrangements fall under the same requirements.
In March 2026, the FCA described firms using data vaulting, immutable backups, standby data centres, and new processing centres to help recover important business services within impact tolerances after cyber disruption. These are examples of investments observed by the regulator, not a universal checklist or a guarantee of recovery. The FCA’s observations also cited 2025 cloud-provider outages and cyber incidents as reminders that third-party and technology disruptions can affect services.
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New UK incident and material-third-party reporting requirements set out in FCA policy statement PS26/2, published on 18 March 2026, are due to apply from 18 March 2027. Organizations should check the policy statement and applicable rules for exact scope and reporting details before making compliance decisions.
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The US Government Accountability Office (GAO) reviewed 69 federal legacy systems in July 2025 and selected 11 it considered most in need of modernization. In that selected group, eight systems used outdated programming languages, four had unsupported hardware or software, and seven had known cybersecurity vulnerabilities. These figures describe that sample, not regulated businesses generally.
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GAO found that only three of the 11 selected systems had modernization plans containing all three elements it examined: milestones, a description of the work, and details on the legacy system’s disposition. GAO warned, “Until agencies fully document modernization plans for critical legacy IT systems, their modernization initiatives will have an increased likelihood of cost overruns, schedule delays, and overall project failure.” See GAO-25-107795.
A separate GAO review of the Navy’s financial-management modernization, published in April 2026, found that the effort fully met one of four assessed migration-planning practices and partially met the other three: an enterprise roadmap, executive monitoring, periodic reviews, and a system to track progress, issues, and action items. GAO also reported at least 111 changes to consolidation plans, including at least 49 system schedule delays. These are findings about that modernization effort, not a typical-project benchmark; they illustrate why oversight and change visibility need to be part of the plan.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should you compare migration approaches?
There is no universally safest architecture or cutover technique established by these examples. Compare candidate approaches against the service and the evidence your organization needs, rather than choosing on deployment speed alone.
- Continuity: How much exposure does the approach create against the service’s disruption limits during each stage?
- Dependencies: Does it reduce, preserve, or add reliance on particular systems, teams, facilities, information flows, or third parties?
- Testing and recovery: Can the organization test realistic failure scenarios and show that recovery meets service requirements?
- Data and audit evidence: Can data obligations be met and can decisions, results, and changes be evidenced?
- Reversibility: If a stage fails, can the organization pause or recover without creating a larger service interruption?
- Governance: Are milestones, decision rights, issue owners, and executive monitoring clear enough to control change?
- Legacy disposition: Does the approach lead to a defined end state, or does it leave the old estate running indefinitely?
Record why the chosen approach fits the service’s requirements and what residual risks remain. The decision should be explainable to the people responsible for service continuity and, where applicable, to the organization’s regulator.
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