I tried to upgrade Win 8.1 Pro several times both through Windows Update and the standalone installer, and it failed each time.
If I start the WU update, WU downloads the files, creates the $WINDOWS.BT folder and then, near the end of the process, displays a failed update screen with the C1420127 error code.
Clicking Try Again immediately displays a "No updates are available" screen.
I tried deleting both the SoftwareDistribution/Download and $WINDOWS.BT folders. I also tried running the Windows Update Fix-It tool. I don't have an AV running at the moment. Nothing worked.
Then I tried downloading and running the Win10 Setup program. It downloaded the files, ran preliminary checks and updates, went full-screen ("Installing Windows 10") and then, when it reached 20%, it shut down and displayed a "Windows 10 Installation has failed" error messages. I tried running it a few more times, including running the setup from a USB drive and nothing worked.