When you go to install an upgrade, the supported upgrade paths are checked in 2 registry Values located in “HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Current Version”
Value 1: ProductName / Value 2: EditionID
If you can look up these values in an existing server, you find that Server 2012 R2 Standard ProductName=Windows Server 2012 R2 Standand / EditionID=ServerStandard
By changing the values that were in there for Foundation to these, I was able to immediately (no reboot needed) launch the Server 2012 R2 Standard setup and it proceeded to perform an in-place upgrade that worked flawlessly.
Hopefully this helps out someone else stuck in a similar circumstance. PS – You can use a similar procedure to upgrade Windows 7/10 Professional to Enterprise