Massimo está certo, mas você encontrará a resposta oficial em KB 951041 :
Upgrade considerations for Server Core installations of Windows Server 2008
- You can use the Server Core installation option only by performing a clean installation.
- You cannot upgrade from earlier versions of Windows to Server Core installations of Windows Server 2008.
- You cannot upgrade from non-Server Core installations of Windows Server 2008 to Server Core installations of Windows Server 2008.
- You cannot convert Server Core installations of Windows Server 2008 to non-Server Core installations of Windows Server 2008.
- You can upgrade Server Core installations of Windows Server 2008
only to Windows Server Core R2 when it is released.