This post is being written before the preview release is out, and before guidance has been written. It is based on what we know from TechEd NA 2013.
Upgrading a non-clustered Hyper-V host has never been easier. Microsoft did some work to increase compatibility of VM states between WS2012 and WS2012 R2. That means you don’t need to delete snapshots. You don’t need to power up VMs that were in saved states and shut them down. Those files are compatible with WS2012 R2 Hyper-V.
There are 2 ways to upgrade a WS2012 R2 Hyper-V host.
Do An In-Place Upgrade
You log into your WS2012 R2 host, shutdown your VMs or put them in a saved state, pop in the WS2012 R2 media, and do the upgrade. The benefit is that you retain all your settings, and the VMs are right there in Hyper-V Manager with no effort. The downside is that any crap you might have had on the Management OS is retained. Microsoft always recommends a fresh install over an in-place upgrade.
Replace The Management OS & Import/Register The VMs
I prefer this one. But be careful – do not use this approach if any of your VM files/settings are on the C: drive of the host – I hate those default locations in Hyper-V host settings.
You shutdown the host, pop in the media, and do a fresh install over the C: drive of the host. This gives you a completely fresh install. Yes, you have to rebuild your settings but that can all be scripted if you’re doing this a lot. The final step is to import the VMs using the register option. This simply loads up the VMs, and then you start up whatever VMs you require.
Upgrading to Windows 8.1 Client Hyper-V
This is a little off-topic but it’s related. You can upgrade a PC from Windows 8 with Client Hyper-V to Windows 8.1. The upgrade will automatically put running VMs into a saved state. After the upgrade, the previously running VMs will be running.
The final step in any Hyper-V upgrade is to upgrade the Hyper-V Integration Components in the guest OS of each virtual machine.