Changing Windows 7 license from upgrade to retail possible?

[Foxhack]

Hi folks. I have a question about changing the license in a Windows 7 system.

I currently have Windows 7 Home Premium Upgrade installed on my PC, which used to have Windows XP Pro Retail on it. What I want to know is, if I get a Windows 7 Professional key (from a retail box, which I'm currently hunting down), will I be able to enter the key into the system and change my OS to Windows 7 Pro, or do I have to reinstall Windows to be able to have 7 Pro installed?

I know that Anytime Upgrade would change my OS into 7 Pro, but I'd rather change the key to 7 Pro Retail so I can sell or trade away my older 7 Home Premium Upgrade and be able to legally make my next system build an XP Pro / 7 Pro dual boot system later on. (Why? I own a lot of classic Windows 95/98 games and some simply won't work on 7, much to my disappointment.)

Any help is appreciated. Thanks!

 

[CrimsonKnight13]

Upgrading with the 7 Pro disc & key will work.

 

[Foxhack]

Phew! Thanks. I really didn't want to reinstall Windows, glad to hear that I won't have to!

 

[Stoly]

(Why? I own a lot of classic Windows 95/98 games and some simply won't work on 7, much to my disappointment.)
Any help is appreciated. Thanks!

¿Care to give some examples?
It has taken me a while and a lot of effort to get most of my gaming library (circa '95) work on windows 7x64 and now windows 8 x64