An old connection may contain rules nobody has written down. Before switching it off, run the replacement against representative records in a test environment or produce comparison output that cannot post live transactions.
Explain the differences
Compare references, quantities and status changes. Each difference needs a decision: an old fault to correct, an intentional change or behavior the new build missed. Keep that decision with the test.
Account for unfinished work
Identify the last old-route record and the first new-route record. Include delayed messages, queued retries and jobs already running. Staff need to know which route is active and who can pause it.
Keep a recovery plan for already-posted transactions. Restoring code does not reverse a stock adjustment or invoice. The repair and replacement service covers this investigation.