Q: Can I downgrade Windows Server 2012 Datacenter to Windows Server 2012 Standard?
A: No. You can't downgrade Windows Server 2012 Datacenter to Standard. Although the two different SKUs are identical, other than via licensing, you can't downgrade from Datacenter edition to Standard edition. This is confirmed via the DISM command below. The output below that command shows there are no available "upgrades."
C:\>dism /online /get-targeteditions
Deployment Image Servicing and Management tool
Image Version: 6.2.9200.16384
Editions that can be upgraded to:
(The current edition cannot be upgraded to any target editions.)
The operation completed successfully.
See more FAQs at John Savill's page. You might also be interested in the FAQ "What Downgrade Rights Do I have with Windows Server 2012?"