If the current motherboard has windows 7 on it and you're planning on changing motherboards, you might have troubles. Most CD-key's (I think only system builder keys are the exclusions) can either be used a very limited amount of times or its a one time use. Meaning that when you activate the key on one motherboard, you cant use it on another motherboard.
If you got the windows 7 key from HP, then its only tied to what ever motherboard you activated that key on.
If you bought windows 7 from a third party such as a best buy or amazon, then it might be ok.
You have a Windows 7 home premium key but a Windows 7 pro disc... (Or thats what I am getting at.)
You'll need to create an ISO on a thumb drive for windows 7 home premium in the respective key type (such as 32 bit or 64 bit.)
I dont know if that answered your question, but that is what it sounds like you need to do. I dont think you can use a home premium CD key on a pro disc install. Especially if that CD key was originally from HP and not from a different source.