I'm about to perform Domain and Forest Functionality level upgrade from Windows Server 2003 into WIndows Server 2012 R2 if possible, but I wonder what are the caveats and pitfalls ?
Single domain AD forest
4x Exchange Server 2010 SP3 OnPremise soon to be migrated to Office 365 in a few months.
2x Windows Server 2012 R2 Domain Controllers FSMO role holder (PDC, RID & Infrastructure master)
6x Windows Server 2008 R2 Domain Controllers (Schema & Domain naming master)
I reckon that there is no roll back plan because this process cannot be rolled back.