I'm planning on migrating 2 HyperV VMs that are currently running on Windows Server 2012R2 to a new server running 2008R2. What's the best way to migrate the VMs? Will replication work between the two OSes? How about the export feature in HyperV? I know that it's not possible to move directly from 2008R2 to 2012R2 without an intermediate conversion, but I can't find any info on doing the reverse.
I would recommend using Hyper-V 2012 R2 on all new installations. If you cannot afford Windows Server licensing for the hypervisors, use Hyper-V Server which is free.
That said, if you are denser than 7:1 it makes financial sense to license Windows Server Datacenter Edition which entitles you to unlimited OS installs on a physical host including the hypervisor OS. A single Datacenter edition license on a two socket host will allow you to run the host OS plus unlimited guests of any Windows Server version