Find a boundary before choosing a framework
The hardest part of replacing an old system is often discovering what it does. Reports, exports and manual workarounds can contain business rules that never reached the documentation. Start with the operators and the outputs they rely on.
Map the systems of record and follow the movement of data. Identify who writes each record, who reads it and which discrepancies people currently resolve by hand.
Replace one responsibility at a time
The strangler pattern moves a bounded responsibility into a new component while the rest of the old system continues to run. The boundary may be a workflow, an integration or a reporting responsibility. Choose it for a reason: independent verification and a manageable rollback.
This approach is not an excuse to run two uncontrolled systems indefinitely. Each phase needs a defined end state and a plan for retiring its predecessor.
Reconcile before switching
Parallel running only creates evidence if the team knows which outputs must match and how exceptions will be resolved. Define reconciliation rules before collecting results. Include timing differences, failed records and work that crosses the cut-over boundary.
An acceptance criterion such as “the data looks right” leaves the decision with whoever is most confident. A named set of reconciled outputs makes it examinable.
Agree the way back
Rollback needs a decision point, an owner and a treatment for transactions written after the switch. Restoring an old application does not necessarily restore the business to a consistent state.
Plan recovery with the same care as migration. Controlled replacement works when every step reduces uncertainty and leaves the business in a state it can operate.