Open up command prompt and type "slmgr -dli". This will show you whether it is activated as retail or OEM. If it says OEM, chances are you will need to buy a new key. If it says retail, it should work.
Note that you might need to re-enter your product key if you are using retail once you change.
Also note, of you are on OEM, you might be able to phone the nice guys at Microsoft, and tell them that you changed your hardware and the key stopped working. Depending on the guy who picks up, they may or may not get it working for you. Just don't specifically tell them you upgraded your MoBo unless they ask.