The worst time to discover a migration problem is after cutover, when the old system is already offline and the target system is live in front of users. A report that used to balance no longer does. A scheduled job that ran fine last week fails silently. Nobody rehearsed the rollback plan, so rolling back isn't really an option anymore.
Data migration testing exists to catch these problems before cutover, not after it. This post lays out a testing strategy across the full migration lifecycle, from validating the source before data moves to proving data integrity after it lands