Windows Server 2012 R2 delivers something to Hyper-V that we have not had before, Cross Version Live Migration. What this means is that VMs running on a downlevel version of Windows can be easily moved to an up level server.
Example 1: Server running Windows Server 2012 with 10 VMs can Live Migrate those VMs to a Server running Windows Server 2012 R2.
Example 2: A cluster running Windows Server 2012 can use the following steps to upgrade to 2012 R2
- Move all the VMs to node 1
- Upgrade node 2 to 2012 R2
- Live Migrate all the VMs to node 2
- Upgrade node 1 to 2012 R2
- Balance the VMs as needed
NO DOWN TIME WHEN UPGRADING! NONE!!