We're going to be upgrading our servers from Server 2003 SP1 to Server 2012 R2 soon (finally), but I wanted to doublecheck that once the Domain Controller is upgraded from 2003 SP1 OS to Server 2012 R2 OS at a 2012 R2 functional level, will the other Server 2003 SP1 machines still be able to join the domain and function properly for a while until they are upgraded as well?
I found this answer:
Join Windows 2003 R2 guest to Windows 2012 R2 domain controller
But it was asking about 2003 R2. I'm asking about the older 2003 Vanilla with Service Pack 1.