Yes i already have Win7 Pro disc
Okay, assuming your disc is:
a) legal, and
b) same bit version (i.e. 64-bit Pro upgrading 64-bit Premium)
I'm guessing you can simply do this:
1) Insert the DVD
2) Follow the UPGRADE path
*Basically it will allow you to upgrade or it won't. I could find NO information on people doing it this way. Unless you have a REASON to upgrade it might be better to just SELL the it while people still want Windows 7 Pro.
Is this an OEM copy (cheaper packaging, will say "OEM" somewhere) or is it a full copy purchased in a store?