I recently upgraded a 2003 DC to 2012R2 DC. I was abele to move all FSMO roles and Schema role to the new DC with out a problem. When I went to demote the 2003 DC it did not recognize the 2012 controller and in turn notified me it was the last DC in the Forest. No big deal, I'll just remove it, except when I shut the 2003 DC off I cannot join anything new to the domain. When i turn it back on I can join new items Domain.