Hi, The company I work for is a smaller company (about 200 end users) and we currently have an AD domain/forest running Windows 2003.
From reading Mr. Minasi's book on Windows 2008 R2 I'm seeing that there are quite a few qualifications as to what severs can be upgraded to Windows 2008 R2. My biggest concern right now is that all my existing Win2k3 servers except for a few are x86 systems. As I understand things, it will not be possible to migrate these x86 systems to x64.
So is it all right to build some new DCs (DNS, GCS) with Win2k8 R2 for the 64-bit abilities but then other member servers could run just vanilla Win2k8? I would install the new DC servers with Win2k8 R2, join it to the existing domain and then slowly demote the existing DCs/DNS I have to just be Windows 2003 member servers in a Win2k8 domain until I could take them fully offline. I know there's also steps for forest prep, domain prep, etc.
I would think as long as the DCs are all the same there wouldn't be any real issues with the member servers being a hodge podge of servers until I can get them all either upgraded to Win2k8 or replaced entirely. All new servers I would try to keep as Win2k8 R2 as well.