Start with a workflow, not a screen count
A legacy application's value is rarely limited to its visible interface. It may enforce approval rules, reconcile records, schedule jobs or compensate for an upstream system's limitations.
Choose one workflow and follow its records from entry to completion. Include reports, background jobs, exported files and the manual checks people perform around the application. This reveals dependencies a screen inventory alone will miss.
Separate rules from implementation habits
Ask why each step exists. Some rules reflect genuine business constraints. Others reflect an old technical limitation or a workaround that no longer has an owner.
Record that distinction without assuming every old rule is obsolete. A modernization plan should preserve necessary behavior while creating a deliberate opportunity to simplify the rest. The people responsible for the process need to validate those decisions.
Define a boundary you can test
A phased replacement needs a clear line between old and new responsibilities. Agree which system owns each record during the transition, how updates cross the boundary and how the results will be compared.
Use representative cases, including exceptions and historical edge cases. Data migration needs reconciliation, not just a successful import message. Keep a record of transformations and decisions so differences can be investigated.
Plan for the day after release
Document support ownership, access, scheduled work, alerts and recovery procedures. A modern interface without an operating plan can become the next difficult legacy system.
The first release should prove a manageable transition pattern. Its results can inform later phases, including whether to retain, replace or integrate other parts of the application.