This one sort of bit me recently. A backup and restore on Exchange to a new server with the same name will NOT work! What you need to do is create the new server with a different name, add it to your current domain, and make it part of your Exchangeserver site. Then move your mailboxes to the new server and test. Then move other data and test. Then, when you're sure everything works, you can take out the old server. But, (and here's the rub) you won't be able to change the name of the new server to the old server's name because they have to be domain controllers for SBS and you can't change the name of a domain controller.
For your own sanity, do this in a test environment first and NOT in production, because Exchange does NOT like to be moved and particularly not the version that's in SBS!
(When I tried to just restore an Exchange backup to a new server with the same name, the Information Store crashed and was unrecoverable by any of the utilities or documents Microsoft postedon TechNet. It was quite ugly. I certainly wouldn't want to do it with 300 users!)
Your life will be much easier if you just write login scripts to change users drive mappings to the new server and give up on keeping the same name for the server.
