Moving what you already hold into the new system, and proving it arrived intact. The part of a rollout most likely to go wrong, and the part most often left to the last week.

Migration is the long pole. We ask for a data extract at the start of a project rather than the end, because what we find usually changes the plan.
Reconciled record by record against what you handed over, with the differences explained rather than absorbed.
Duplicate records, missing fields and the entries somebody typed into a notes box get surfaced while there is time to decide what to do.
The new system opens with your real history in it, so nobody has to keep the old one running alongside.
We take your data and tell you what is actually in it, which is rarely what the field names suggest.
Duplicates, blanks and contradictions are listed and put to you. We do not quietly pick a winner.
Run repeatedly into a test environment until the reconciliation comes out clean, before production is touched.
Counts and totals compared both sides, with a written record you can hand to an auditor.
You give us a copy. We do not need write access to your old system.
What is in there, what is wrong with it, and what it means for the plan.
Decisions on the messy records, and a field-by-field map into the new system.
Into test first, repeatedly, then production once the numbers tie.