With Windows Server 2012 R2 Hyper-V, you will able to easily migrate your virtual machine from Windows Server 2012 to R2 without causing any downtime. You can perform cross version live migration
But do take note that you cannot perform live migration from Windows Server 2012 R2 to 2012. Moving a virtual machine to a down level server running Hyper-V is not supported.
This error message will appeared when using
a) VMM 2012 R2 (Preview Edition) – to live migrate from Win 2012 R2 to Win 2012
b) Windows Server 2012 R2 Hyper-V (Preview) - to live migrate from Win 2012 R2 to Win 2012