Classify before moving
Separate active operational records, reference data, attachments, history and records that can be archived. Each class has different validation, retention and cut-over requirements; treating everything alike increases cost and hides risk.
Preserve business meaning
For every important field, record its definition, owner, allowed values and known exceptions. A technically valid date, status or customer number can still be wrong if its meaning changed between the source and target system.
Validate meaning, ownership and totals on both sides of the move.
Plan cleansing and reconciliation
Decide which errors are corrected in the source, transformed during migration or accepted with an exception record. Reconciliation should compare counts, totals and selected business cases rather than rely on a successful import message.
Rehearse the cut-over
Run at least one timed rehearsal with representative volume, issue logging and rollback steps. The final plan should name the data freeze, responsible people, validation window and decision needed to release the new system.