Hi David, welcome to the Ten Forums.
In Windows 7 it was and is important to generalize the Windows installation before transferring it to a new PC or the same PC when hardware will be replaced. For this I have written a tutorial on our sister site the Seven Forums: Windows 7 Installation - Transfer to a New Computer - Windows 7 Help Forums
However, although the method in that Windows 7 tutorial also applies to Windows 8, 8.1 and 10, they are much better in coping with the hardware changes and usually you don't even have to generalize Windows before switching a motherboard like you did. Using the Sysprep generalizing is in any case a sure way to make Windows to work when replacing the motherboard. That being said, you have done right in using the method but sadly you have also done something wrong; the issues you told about do not occur when generalizing is done correctly.
To show you the workflow in Windows 10 I repeated the process now in a virtual to check it really works in Windows 10 as I have told in that Windows 7 tutorial. Here's how it should be done in Windows 10:
1. Close all programs and applications, run the sysprep command with generalize, oobe and shutdown switches:
When sysprep has shut down your PC, replace the motherboard
Start PC normally, it will now go through the OOBE / Welcome mode
Create a temporary local
Let Windows prepare the user profile for this temporary account, sign out as soon as you arrive to desktop
Now select your old, original admin account and sign in. Notice that because you have generalized Windows, it needs to go through a short user profile setup process as if your old account would be a new account. However, when setup is ready and your old account desktop is shown, you'll notice that all your data is there; Windows recreated your user account using the same username, user profile folder and data.
When signed in with your old, original user account simply delete the temporary user you had to create in step 4 (tutorial
That's it. Your original user account will now work exactly as it did before the generalizing. If the PIN sign-in is not working, just remove the PIN and set a new one.