Windows Licensing with SPLA license

Hi there, 

We had a Windows 2012 R2 Datacenter Edition VM on Azure created with the default image that Microsoft provides for this purpose. We have downloaded the VM to our on-prem Hyper-V cluster and we are trying to "re-license" the VM using our SPLA licenses. The Azure license key that comes with the machine has been deleted but the machine won't take any of our Windows 2012 R2 SPLA keys, neither the Standard nor the Datacenter. 

Any thoughts?

Thanks

Sandor