This way is probably not supported, and I would not try it on something like a DC... but here is what I recently did to downgrade Server 2012 R2 Datacenter to Server 2012 R2 Standard.
Edit: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion
Key: EditionID
Change To: ServerStandard
Key: ProductName
Change To: Windows Server 2012 Standard
Note that in this example I am going to be "upgrading" to Server 2012 R2 Standard, so I put in Windows Server 2012 Standard as the ProductName, not Server 2012 R2 Standard. After the upgrade, this key will change itself to proper version.
Key: EditionID
Change To: ServerStandard
Key: ProductName
Change To: Windows Server 2012 Standard
Note that in this example I am going to be "upgrading" to Server 2012 R2 Standard, so I put in Windows Server 2012 Standard as the ProductName, not Server 2012 R2 Standard. After the upgrade, this key will change itself to proper version.
Next mount or insert your Server 2012 installation media, run the installer. Do not check for updates, I chose Server 2012 R2 Standard with gui as my OS, and pick Upgrade.
It will go through the standard upgrade process, and you now have downgraded from Server 2012 R2 Datacenter to Server 2012 R2 Standard.
After that I ran slui 3 from command prompt, entered my Server 2012 R2 Standard product key, and activation went through.
I'm about to test doing this on a DC, but honestly if you were to try this, I would demote the server, remove DNS and DHCP, then do the "downgrade".
