I've done it a few times. Stop the service, create DB backups, restore the backups (making sure you put the mdf / ldf files where you want them), deal with the permissions (don't forget about MAS_Reports) and use the Sage utility to re-point the service to the the new instance. Any external connections will have to be moved too (silent DSN's, excel queries...). Once tested we disable the old SQL instance when we do this to force people to use the current database.
If something goes wrong, the old SQL instance is still there, and you just reverse the Sage settings to point back to the original server.