I had the identical problem. The problem is that the Windows Update Agent is out of date and does not update itself first as it is supposed to do.
To fix this you first download the most recent Windows Update Agent for your OS:
Note: Microsoft support kept referencing an outdated page for kb949104 which is NOT the most current WU Agent.
To update the Windows Update Agent:
- Open an elevated command prompt.
- Stop the Windows Update Agent by typing
net stop wuauserv and hit Enter.
- Run the installer you downloaded earlier.
- Start the service by typing
net start wuauserv.
- Restart your computer.
Windows 8.1 should now update itself and then you can run the W10 update. It may take a few days and a few restarts for W8.1 to fully update but you should start seeing results in the Window update history file.
You can try manually installing the updates but i found it best to just leave the machine on for a couple of days and restart as requested, and let it do its own thing.
It would be very nice if the Windows Update Agent actually updated itself, but it doesn't sometimes, and so you have to go through all this to make it work the way you expect it to.