As pantscat says above, Exchange makes things a bit more of a pain. A fresh /new forest is definitely the way to go, much cleaner and less hassle. The big thing though is in the testing! Setup a couple of servers, on an isolated switch, setup Exchange with some test mailboxes, setup a server to represent what will be your new domain, and a new Exchange server, and test (and re-test) the mailbox migration. Might be worth checking the SQL setup for SIMS too, as that may well need re-permissioning. If SIMS is supported by a third party, definitely get them onboard (easier said than done, but easier to plan up front than fire fight if SIMS doesn't play nicely after the migration). The upheaval (if that is the right word) is worth it in the end, a new clean 'known quantity', without any left over policies, permissions, etc.