For those good and proactive server admins who are working hard to migrate Windows Server 2003 installations before the end of life deadline on July 14, 2015, you might want to hold off for a bit – at least until Microsoft produces and releases a hotfix.
Microsoft has revved up the end of life notices for Windows Server 2003 just recently, and the company is truly wanting to do a better job of warning customers than they did with Windows XP, but they've hit a snag. Obviously, Microsoft wants businesses to run its latest server operating system, Windows Server 2012 R2, but a severe issue has arisen when both Windows Server 2003 DCs exist on the same network as Windows Server 2012 R2 DCs.
Microsoft realized the issue due to a higher than normal volume of support calls from customers with issues while migrating from Windows Server 2003 to Windows Server 2012 R2.
Microsoft has provided a workaround and is intent on delivering a hotfix, though it's stated that "it’s going to take us some time to get it out to you."
Details on the issue, and the suggested workaround is wrapped inside an apologetic blog post here: It turns out that weird things can happen when you mix Windows Server 2003 and Windows Server 2012 R2 domain controllers