The OEM copy is restricted by license (although any technical restrictions are probably weak at best) to the computer it was originally purchased with. The Windows 8 license has no such restriction. Its license only requires that a previous copy of Windows be installed, and the license of that previous copy is unimporant. However, the edition (Pro, Home, etc) of the previous version is important, but the Pro edition of Windows 8 can upgrade just about anything so you're unlikely to run into any problems there. You can roll back again later if you want, but you'd have to do a fresh install as there is no downgrade feature.