I have an associate who has a SBS 2003 server / network running Exchange 2003. He's purchased a new HP server with Windows Server 2012 (Standard). He was intending to use this to completely replace his SBS box. Because of an issue with one application running on his SBS box he needs to keep it running for around a year before it can be decommissioned.

The 2012 server has already been completely setup with Exchange Server 2013 and as an Active Directory / DNS server for his external domain (company.com). His SBS box is currently setup for all of this, too, but with his internal domain (company.local). I've come to the conclusion that we're going to setup split DNS so he can use both boxes now and take advantage of the new hardware, new OS, and new version of Exchange.

What I'm wondering is if it's best practice to just demote/remove Active Directory / DNS from the new box, leaving Exchange 2013 on there, then joining it to his existing 2003 domain...

- or -

should we blow away the new box and start over from scratch? I'm mostly concerned about the Exchange 2013 installation and if demoting/removing all of the Active Directory and DNS stuff and then joining it to a completely different domain will completely throw it off and break it.

Thoughts?