First of all, We already have a server as a DC running Windows Server 2008 R2, So I've obviously run adprep /forest prep on my Schema Master.
But, when trying to promote another Windows Server 2008 R2 Server, DCpromo said I should run it. So I did run the version from that particular disc on my Schema Master, and it did indeed update the schema by a couple of versions. HOWEVER, dcpromo is still demanding that I run adprep /forestprep and refusing to promote the server to a DC.
The schema master is running Windows 2000 Professional Server.