Build it, try it. If it does not work, call Microsoft and tell them you had to replace your motherboard and they should give you a new key.
I think windows 7 had a way for a OEM to put a BIOS passkey in. Windows itslelf did not. Problem ways pirates grabbed the key and put it in machine BIOS to get out of getting a license. I think Window 8 changed that method of validation for OEMS to require unique keys. I don't know if a upgrade would actually write a key to the BIOS on a general machine. Too many problems, upgrade your BIOS and you no longer have a windows 8 license? That would be bad. I would think a OEM would burn a ROM location with a key so it would not get wiped out later and would stay with the machine