Hi,
Do you have your Windows 10 product key? or are you using a Digital License which is tied to your Microsoft account?
if it's a product key, find it first:
https://support.microsoft.com/en-us/help/10749/windows-10-find-product-key
disclaimer: some OEMs might not allow you to reactive a preinstalled OS once you attempt a major hardware change such as motherboard.
if you still wanna go on, here is the way:
the best way is to first delete your Windows 10 key from your current Windows 10 installation, then swap motherboards and reactivate your OS again.
1. Disable Secure Boot in BIOS
2. Open an Admin Command Prompt:
3. slmgr /upk (uninstalls the key)
4. Press Enter
5. slmgr /cpky (deletes it from the registry)
6. Press Enter
7. Restart system
Check is your system is not activated with the key(it might take a while for Microsoft's servers to register the change, so wait).
Now before turning off your computer and going for hardware replacement, make sure to Format your C drive (Windows installation drive) before moving forward. this step in necessary.
You can do so but putting a Windows 10 installation media in your computer (DVD or flash drive) and then do the format.
That's it, now upgrade your hardware.
after you're done, reinstall your Windows 10 then you can follow normal procedures to active your OS again.
If second activation cannot be done automatically online, you can always contact Microsoft and activate by phone.
The numbers are displayed on screen during the phone activation where you can select your country
https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-change
The requirement to comply with the EULA is only that the Windows installation be removed from the old computer before reusing the license on the new computer. When the old motherboard is disconnected from the hard drive/SSD, Windows has been removed from it.
