Hi Steven,
I am Sumit, an Independent Advisor and a 3 year Windows Insider MVP here to help.
Use the following generic key to upgrade to Windows 10 Pro for workstations:
DXG7C-N36C4-C4HTG-X4T3X-2YV77
As you changed the motherboard, you would have to run Activation troubleshooter.
https://support.microsoft.com/en-in/help/20527/...
Select I recently changed hardware on the device.