I just completed an experiment for Bruce... One of my existing members joined our new empty group here, with the same email address, before the import. Bruce wondered it she still retained ownership of her messages AFTER the import, so I asked her to edit a message from 2016 in our newly imported group.
Voila! She was able to do so. Ownership was retained!
Also, her edit was sent for re-moderation and approval to post.
Hopefully, our little test will allay one worry about moving members and getting going here while awaiting import.?
Juulz