From WHMCS or BILLmanager. Your current system stays readable. Finances are reconciled with zero tolerance before anything switches. The whole engagement runs under the DPA.
The source is read only. Every record arrives with its identifier, then finances are reconciled to the cent.
We count the data and record discrepancies.
A full migration into a test environment.
Zero tolerance is the condition to continue.
Downtime window measured, rollback agreed.
Final run and an engineer on duty.