You will probably have call Microsoft and explain to them that you got a knew motherboard and it is not activating. If that is the case they will give you a new key to activate it. I had that problem when I changed out motherboards in mine.
Sonny is correct. After a hard drive or motherboard change, you have to call Microsoft and they will give you a new key. There is a limit to how many hardware changes to a computer. Many years ago, l had a desktop computer and changed the hardware many times and they said l was over the limit and ended up going to a computer store and plunked down 300 dollars for a new windows 7 professional disk with key.
Just a follow-up... If I swapped my current system drive to my new PC, will it check and change the system drivers automatically or must I do a clean install of windows 10, and reinstall all my other applications again?
I built the new PC and moved the ssd from my old system to the new. Windows 10 started right away unactivated as expected, but I was able to activate it again (online) without a hitch. I then updated with new MB drivers. Works great!