Data migration is the work of moving records out of a system you are leaving and into the one you are adopting: mapping fields between them, cleaning what will not transfer cleanly, and reconciling both sides so the numbers still agree afterwards.
Let's TalkRecords, relationships, and dates arrive intact, so the new system starts with the context the old one had.
Both sides are reconciled before anyone signs off, so the migration ends with evidence rather than optimism.
Duplicates and dead records are dealt with on the way through, because a migration is the cheapest chance you will get to clean them.
The move is rehearsed before it is real, so the day it happens is a repeat rather than a first attempt.
This is one part of our enterprise software implementation work. Engagements rarely stay in one box: a platform rollout turns into something custom, and both then need looking after. That is why our software development and implementation practice treats implementation, custom development, and ongoing support as one continuous piece of work rather than three separate vendors.