I have an upgrade disk for windows 7 Pro, but I need to install on a blank machine so I'm looking at buying a full retail version. My question is, if I buy win 7 home premium, can I upgrade to pro with my other disk?
Yes, you can, however this is a very expensive way of doing it.
This being said, if you have any edition of Windows XP or Vista (Not OEM) that you are legally allowed to install on this machine, this can count as your previous version of Windows - then using your Windows 7 upgrade disk, you can do a fresh/custom install.
-- Just checked, you cannot use anytime upgrade - the fastest way to change editions as you need a separate anytime upgrade key and not just a new key.