Why you may not want to do an inplace upgrade of a hyperV box to 2012
Unable to convert to Server with a GUI from Server Core on an upgraded Windows Server 2012 machine:
Assume you have a full installation of either Windows Server 2008 or Windows Server 2008 R2. You upgrade to Windows Server 2012 and choose option "Server with a GUI".
After the upgrade you convert the Server with a GUI installation to Server Core.
In this scenario, if you try to convert back to Server with a GUI, the operation may fail and rollback to Server Core.
* The "Server with a GUI" option is the Windows Server 2012 equivalent
of the Full installation option available in Windows Server 2008 or
Windows Server 2008 R2.
* In Windows Server 2012, An administrator now has the ability to
switch between a Server with GUI and a Server Core, as needed. You
convert an installation from Server with GUI to Server Core.
This problem occurs due to the following three unused WINEVT registry entries being carried over by upgrade: