I've moved 1/2 of my lemm.ee communities to a new instance. It was a tremendous amount of work because I like to have archive posts that can be referenced from inside of lemmy (so had to repost the reference material).
In the future lemmy would benefit from a community backfill mechanism (kinda like piefed, but also with refederation of the backfill)
In the future I would change how I backfill posts, I'd make blank posts with a title like [backfill] or [blank], get them federated for a day or two... then edit the posts for the backfill content. This would be less of a burden on users browsing by NEW getting a absolute tidalwave of archival content they may not like
My migrated community already got a YPTB post (nutrition communities bring out the passion in people), it will probably take a week or two until everything settles down.
Reimporting ban lists into a new community could also benefit from automation.