Here are the steps to activate WIN10 on your new PC.
1. Log onto your Microsoft Account
2. Get your copy of win10 on your PC linked into your account. You will see it as a digital Entitlement version of windows. (Select the Start button, select Settings > Update & security > Activation > and then select Add an account. You must be signed in as an administrator to add your Microsoft account.)
3. Swap the CPU and MB
4. Boot. If it fails do a 'refresh' to reinstall windows and keep your current data. Worst case, reinstall windows from the free download created by the win10 media creation tool.
5. Windows will not activate. Go into Activation troubleshooter (Start button, select Settings > Update & security > Activation , and then select Troubleshoot. )
6. From the list of devices that are linked to your Microsoft account, select the device that you’re currently using, select the check box next to This is the device I’m using right now, and then select Activate.
This will work for a fixed number of times, then the digital entitlement will no longer let you do this, so do all the major changes you need before doing this.
Here is a link to the Microsoft Pub that explains this process:
https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-change
PLEASE post your results if you follow this procedure. I have not needed to replace a MB since win10 appeared, so cannot verify that a win7 --> win10 digital entitlement works when following this procedure. And similar things working for me with XP, VISTA and WIN7 do not mean that win10 works, I am drawing my information from the microsoft page that is linked. Good luck.