Holdum333 is correct. The Windows 10 upgrade licence is non transferable even if the licence you upgraded from was retail.
The Windows 10 licence is for the life of the machine only and Microsoft considers a new motherboard, a new machine.
You can call Microsoft and explain that you had to change the motherboard for whatever reason and they may, as Gary say's, give you a new key but I wouldn't say it was a definite.