Questo forum non si occupa di licensing quindi per avere una risposta ufficiale dovresti contattare il supporto commerciale Microsoft.
In ogni caso secondo questo documento
http://download.microsoft.com/download/F/3/9/F39124F7-0177-463C-8A08-582463F96C9D/Windows_Server_2012_R2_Licensing_Datasheet.pdf non dovrebbero esserci problemi ad eseguire un downgrade mantenendo le due licenze virtuali.
If I want to use the software from an earlier Windows Server version or edition, what are my options?
A: If you have Windows Server 2012 R2 Datacenter edition you will have the right to downgrade software bits to any prior version or lower edition. If you have Windows Server 2012 R2 Standard edition, you will have the right to downgrade the software to use
any prior version of Enterprise, Standard or Essentials editions. The ability to downgrade does not change the licensing or support terms in which you can use the product; the purchased product (Windows Server 2012 R2) rights apply. This means
that the license will continue to cover two physical processors and the virtualization rights do not change. In addition, the Client Access License (CAL) corresponding to the purchased version will apply, as an exception; if you are running a previous version
of one of the eligible downgrades you can use the corresponding CAL version if one exists.
