The "shortcut" to "having to unjoin my workstations ... and joining them to the new domain" is to preserve the domain. There is no shortcut. Theoretically you could arrange some kind of baroque scripted solution to disjoin the machines, sit in a holding pattern until the new domain exists, and join the new domain, but that would be a lot of work for a small number of computers.
There's nothing that can't be fixed in your current Active Directory domain. Every case where I've been called-in to look at a "mess" in Active Directory has turned out to be fixable.
As such, I try to preserve existing domains whenever possible. It presents the most continuity of user experience (leaving their profiles intact and not requiring any trickery like "User State Migration Tool") and is typically the least amount of work. If it's well-planned you could pull the whole thing off w/o any changes to client computers at all. Replacing a Windows Server 2003 SBS file server with a Windows Server 2012 file server in that environment is an afternoon project.
This'll do the trick for you re: removing Exchange: http://support.microsoft.com/kb/833396 Basically, get rid of all your mail-enabled users, groups, etc, then uninstall Exchange.
I'll preface by saying that I haven't done this exact migration before (not to Server 2012), but everything I'm reading makes it look like a migration to Windows Server 2008. Were in your shoes I'd:
- Update the AD schema (
- Promote the new server to a Domain Controller (DC)
- Install DNS on the server server and configuring both machines to the new server for DNS once replication completes
- Uninstall Exchange from the old server
- Copy over all your shared files and recreate the shared folders on the new server
- Transfer the FSMO roles and demote the existing server
Once you've got the old server demoted you can disjoin it from the domain, remove all references to it from DNS, and use a DNS alias to present the new server with the old server's name. If you recreate your shared folders the same way none of your Folder Redirection Policy, profile paths, user-created shortcuts, etc, will need to change.