Migrate in pieces, not all at once, and overlap the two systems rather than detonating the old one. The hard part is not the data — it is alignment on definitions across every department that touches a record. Sequence: inventory, field map sign-off, staged migration, overlap, then decommission.
Two structural rules: a greenfield build can go straight to production, but a merger or a live-data cutover needs a sandbox. And talk to every department, not just sales, marketing and CS — finance, support and product often hold integrations nobody listed.
Every claim above comes from work LeanScale published. These are the sources.