Diamond Fire, Inc. All of your IT needs in one place!

You can get the Bios serial number from a hyper-v vm the same way you would get the service tag from a pc using the following command. 

wmic bios get serialnumber

You get a number like:
0000-0000-0000-0000-0000-0000-00

This number should remain constant. how ever i do can't confirm this as i have not tested what happens when the VM is moved from hyper-v server to another or even to another vm platform like virtual box. 

Method for conversion

1.VirtualBox

Extract .vmdk from OVA using 7zip.
Install Virtualbox
Run from command line : VBoxManage.exe" clonemedium --format vhd infile.vmdk outfile.vhd

or for older versions of Virtualbox

VBoxManage.exe" clonehd --format vhd infile.vmdk outfile.vhd

2. qemu-img for windows
Great command line tool to convert from ova to vhd or vhdx

3. CloneVDI
There is a free GUI program out there called CloneVDI that does not require any command line and has a few extra options. Did not used this one yet.