Is it A) a full retail copy of 7 or B) an oem ($99)/came with the pc? If A then you're good. If B, you can either call MS and beg for a break or buy a new oem copy. B-types are locked to the mobo they were installed on.
If you own the copy of Windows, try calling Microsoft and tell them you are replacing the mobo (failure reasons, damaged, etc. are legit reasons after all you own it) they'll ask for the numbers on the sticker (usually on the side of the tower) and will have you run through some steps that can marry the version to the new mobo. Google is your friend btw if that doesn't help.