A: For Windows Server 2008 and Windows Server 2008 R2, the choice between Server Core and a Full Installation had to be made at installation time and couldn't be changed without reinstalling the OS.
In Windows Server 2012, the graphical shell and the management infrastructure are features that can be added and removed at any time, requiring only a reboot making it easy to switch between the Server Core and Server with a GUI configuration levels (Server with a GUI is the equivalent of Full Installation in Windows Server 2008 R2).
Even though it is easy to switch between configuration levels in Windows Server 2012 it isn't possible to make the switch as part of an upgrade from Windows Server 2008/2008 R2 to Windows Server 2012. You must upgrade to the same configuration level.
- Windows 2008/2008 R2 Full Server -> Win 2012 Server with a GUI only (cannot go to to Server Core)
- Windows 2008/2008 R2 Server Core -> Win 2012 Server Core only (cannot go to Server with a GUI)
Once the upgrade is complete, it would then be possible to add or remove the graphical shell and management infrastructure as required to get to the desired configuration level.