Upgrade 2000 domain to 2003 domain with Exchange 5.5

We are upgrading our Windows 2000 domain to a Windows 2003 domain, and also upgrading our Exchange Server from 5.5 to Exchange 2007. We have two new servers that with Windows Server 2003. If we add the Windows Server 2003 to the 2000 domain and then make it the domain controller, what problems might this cause with the Exchange server 2007 upgrade as opposed to just creating a new 2003 domain and moving our 75+ workstations and user accounts over to the new domain?