Whilst installing a server 2012 R2 KMS key on a Server 2008 R2 KMS host I ran into the following issue:
Error: 0xC004F050 The Software Licensing Service reported that the product key is invalid.
Even though I installed the KMS hotfix kb2757817 for Server 2012 support this was not enough to support Windows 2012 R2. To be able to activate both Windows 2012 and Windows 2012 R2 you will also need to install KMS hotfix kb2885698.
Providing you install the correct hotfix you should be fine