A few tips ago (actually a few months ago) I mentioned the Cross Version Live Migration in Windows Server 2012 R2.? This allows virtual machines to be live migrated from Windows Server 2012 to Windows Server 2012 R2.? Another use of this functionality is ‘Cross Version Save Restore’.? That is, you can put a Windows Server 2012 virtual machine into a saved state, then move it to a Windows Server 2012 R2 Hyper-V system, and finally bring it back into a running state.
This would be quicker than a live migration, but it would mean down time for the virtual machine.
Another use to this would be a rolling upgrade for the physical server.
Put all the VMs into a saved state
Upgrade the host to Windows Server 2012 R2
Restore all the VMs to a running state