Migration is mostly a clean-up exercise. Loading data is quick; what takes time is deciding which records matter and making them consistent. Work through this list before you send anything across.
Decide what needs to come across
Clean before you send
What can usually stay behind
Very old results, settled balances from past sessions and long-gone staff rarely need to move. Keep a read-only archive of the old files somewhere safe. You will seldom open it, but you will be glad it exists.