If you have only a few users, then it is easiest to just disconnect them from the old domain and reconnect them to the new domain. Remember though that you have to create all new user accounts (even though the new accounts have may have the same name, they will have different SIDs and therefore will be considered different by Windows). This may mean that users lose some settings and/or data. A good backup strategy will be important!
However, if you have more than a handful of users, if you have lots of data or email to migrate, if your users value their settings and data or if you need to keep your Active Directory organization intact, then you need to perform a server migration. Microsoft has a fully documented migration procedure, however I know from bitter experience that it doesn't always run smoothly. I would strongly recommend that you visit www.sbsmigration.com and investigate their Swing Migration process. If you speak to Jeff Middleton (Small Business Server MVP), tell him I sent you.
You can follow some of my experience with the Microsoft migration process on my blog, where there is also a book review of a book you may find useful.