Changing motherboard and cpu is a major change that usually requires the purchase of a new license as your old license is connected to the old mobo. The only way to reactivate your old key on your new hardware is to call Microsoft service and explain the change. They may be able to transfer your key to the new hardware.
Results may vary as I never had to do that....
Grizzly is correct. However this has nothing to do with a product key. Windows will setup with the drivers needed to operate the hardware for which it is installed on. When you change hardware. Windows will no longer know how to operate. That is until the correct drivers are loaded. I personally only know of one way to do this. That would be to run Windows installation media again. I've always done fresh installs with new hardware. I've never considered in-place installs. I don't even know if it is possible. Either way product activation is completely different from driver issues. Chances are though you will be faced with both.
I put a blank HDD in a Dell Precision desktop today, booted to the USB drive made by the MCT process, installed and activated since it had Win10 installed on the drive that failed. Only driver I had to go after was for the nVidia card in it but was found by Windows Update.