Create a behavioral baseline
Build an input/output corpus with golden reports, files, database effects, protocol exchanges and complete workflows. Preserve starting state and environment so tests can be repeated. Identify which integrations must remain compatible during transition.
Replace incrementally
- Choose one workflow or interface boundary.
- Run old and new implementations against the same cases.
- Compare results and side effects, not just screens.
- Operate in parallel where safe.
- Retain rollback until acceptance evidence is complete.
See requirements recovery, replacement execution, behavior analysis and the pillar.