SlightlyEdible :
So I built a PC and want to upgrade the motherboard. I purchased an OEM copy of Win 8.1. When I upgrade my motherboard and perform a reinstall of 8.1, will I be able to use the same product key?
No, you will
NOT be able to use that key in most cases. You can try activating it by asking a microsoft representative directly, based on your issue, but since this is a simple upgrade most likely they will refuse and have you buy a new key.