Point the domain back at the old email old, connect to the mailboxes again in Outlook and do a PST backup before configuring the new mailboxes on the new host. Then when you have your empty mailboxes in Outlook you can import the PST.
On the whole was OK. Had a few issues and if you did certain things, you would totally break the database requiring their intervention, but I suspect that's all been fixed now.
I believe you can schedule the SIMS import.