Windows 7 home premium to Pro

I currently have two desktop computers at home.

This computer has windows 7 home premium x64 installed.
I have purchased a w7 pro x64 OEM CD and key, and I want to upgrade this computer to pro.

My question is what happens to my home premium key once I do the upgrade, as I would like to upgrade my second machine running XP, to windows 7 home premium, using the key that is on this computer.

I was told the old key dies with the current machine once I upgrade. I have just spent money purchasing two w7 OS, why cant I re-use it on another machine.

Can someone please advise & enlighten me :P

Thanks.

The home premium- did it come preinstalled or has been installed using a OEM system builder license? See, OEM licenses whether preinstalled or system builder are tied to the first motherboard they are used on, they arent transferrable.

Feel free to ask.

I built the system and installed using OEM cd. I was not aware that the op system would be bound to the pc MB

Is there anyway around this apart from spending more money to get another w7 upgrade?

Why dont you just let the home premium be and upgrade the XP to pro? There are no hardware requirement differences and everything you can do with pro, you can do with home premium too except a few things like joining a domain and XP mode.

Thanks for your help. I think that will be my solution :)