Why is a migration the best moment to redesign?
Because it is the one time everyone accepts that things are going to change. Redesigning a pipeline in a live CRM means managing the team's resistance; doing it during a migration is part of the project and nobody argues.
Whoever replicates the previous configuration as-is loses that window and ends up paying twice: once to migrate and again, months later, to fix what they already knew was wrong.
Which history is worth bringing?
Less than people usually ask for. The natural impulse is to migrate everything, but old history is rarely consulted and makes the migration directly more expensive: more records to clean, map, and validate.
A practical rule: closed deals from the last two years and contacts with any interaction in the last three. The rest gets exported and archived outside the CRM, where it stays available without dirtying the new system.
If the migration is already decided, agreeing what comes over and what gets archived is the first thing we do in an implementation, before touching a single record.