It probably won’t come to this as I have now convinced the management to allow me to purchase Vsphere Essentials Plus, but I was curious about whether I could convert my Windows Server 2012 Standard Server to Windows Server 2012 Datacenter without having to do a complete reinstall.
Good news! It is possible and is dead simple to do. Via http://technet.microsoft.com/en-us/library/jj574204.aspx
From an elevated command prompt run the DISM tool and pop your new key in.
DISM /online /Set-Edition:ServerDatacenter /ProductKey:[Datacenter key, e.g. XXXXX-XXXXX-XXXXX-XXXXX-XXXXX] /AcceptEula