Microsoft has released an update to existing KMS servers. (KB2885698)
This update extends the Key Management Service (KMS) for Windows Vista, Windows Server 2008, Windows 7, Windows Server 2008 R2, Windows 8, and Windows Server 2012 to enable the enterprise licensing of Windows 8.1 and Windows Server 2012 R2.
KMS provides support for the following KMS client activations:
- Windows Vista
- Windows Server 2008
- Windows 7
- Windows Server 2008 R2
- Windows 8
- Windows Server 2012
- Windows 8.1
- Windows Server 2012 R2
So in order to activate your Enterprise versions of 2012 R2 servers and Windows 8.1 you need this update and a new KMS host key with support for Windows 8.1 or Windows 2012 R2.
Installation is straight forward:
And after installation you can follow the instructions in the link on how to install new KMS host key and how to activate the key.