Use a key finder:
Here are four utilities, all of which will may display your product keys if they are still there:
(It does a good job of providing a wealth of information.)
http://www.majorgeeks.com/download4138.html which has additional features
Go to the order summary page, there you will find your product key:
You can download it from there and use your product key to upgrade:
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when you install WMC your product key is changed to the WMC key. the win 8 pro key is no longer there.