Q: Can I Live Migrate from Windows Server 2012 R2 to Windows Server 2012?

A: Windows Server 2012 R2 allows a Live Migration of a virtual machine (VM) from a Windows Server 2012 Hyper-V host. This allows organizations to migrate to Windows Server 2012 R2 from Windows Server 2012 without affecting the availability of the VM during migration. It is not possible to Live Migrate from Windows Server 2012 R2 back to Windows Server 2012.

If you needed to go back from Windows Server 2012 R2 to Windows Server 2012, then you would need to take the VHD/VHDX files from the VM and create a new VM on Windows Server 2012. You cannot export from Windows Server 2012 R2 and import on Windows Server 2012.