Licensing for Guests OS

Hello, 

We are planning to move to Windows Server 2012 DataCanter Edition, and we are aware it suppports unlimited guests OS and it will provide us also free multiple Activation keys, what if we install Windows Server 2008 R2 edition and Windows Server 2003 R2, the  free activation keys will work with these or we need to hae  separate licenses for these versions, does it mean the free activation keys will work only on Server  2012  Edtitions alone?.

Thanks