I like the idea of creating a new database and moving the users to it because that would cause less disruption but I was just trying find out the impact of this approach on the SBS because I would suspect it will break the SBS tools for storage management? Can a new database be created, users moved to it, then the SBS default database be taken offline, defragged and mailboxes moved back?You can use EMC. You need to create a new database, move mailboxes, unmount and drop the old database. There is no need to defrag the old database and move back mailboxes.
Is space becoming an issue? If it is, then you might have other issues that need addressing before you worry about Exchange such as WSUS and SQL as they also chew up a ton of space between them on SBS.
Some light reading for you: