Q: Our Domain is Windows 2012 R2 wit 2003 function level. If we upgrade to Windows 2012 R2 function level, does that cause any problem on Windows 2008 servers?
A: That should be fine. Quote from Microsoft article:
Windows Server 2012 requires a Windows Server 2003 forest functional level. That is, before you can add a domain controller that runs Windows Server 2012 to an existing Active Directory forest, the forest functional level must be Windows Server 2003 or higher. This means that domain controllers that run Windows Server 2008 R2, Windows Server 2008, or Windows Server 2003 can operate in the same forest, but domain controllers that run Windows 2000 Server are not supported and will block installation of a domain controller that runs Windows Server 2012. If the forest contains domain controllers running Windows Server 2003 or later but the forest functional level is still Windows 2000, the installation is also blocked.
