We have 4 Domain controllers running over 100 GPOs which are all Virtual Machines running on Windows 2012 R2 Hosts. I was just wondering if there's a way to directly upgrade the Domain controllers from Windows 2008 R2 to Windows 2012 R2. There are over 50 servers in the farm, including an RDS Farm. Would it break anything if I perform a direct upgrade? How would it affect the remainder of the servers in the farm that are joined up to the domain? I just need to know if there's a way to upgrade, or even a TechNet article that covers how to do this, and whether it's recommended. Again, everything is virtualized, so they are all VMs. Is it easier to do a clean install of Windows 2012 R2? If so, how would the functional level be configured, and at what point does the functional level need to be raised to 2012? Thanks!