Do you have a retail box copy or an oem copy? If you have a Retail Box Copy then you can reuse the same key. If you bought an OEM copy like the one Newegg sells then your going to have to buy a new one.
I replaced my mb twice with an oem and had no problem. Just do the phone in active to microsoft and tell them that your old board went bad and they will give youa series of numbers to enter so that your key works. Good luck.