Windows – any way to recover the original Windows key used for a fresh – upgrade install of Windows 10

I read through the log files in C:\$Windows.~BT and discovered an error message that said:

Unable to create temp file in C:\Users\me\AppData\Local\Temp. The file already exists.

(That's not the exact wording, but it's pretty close. I'm typing this on my other machine while watching the installer run.)

I ran Disk Cleanup on the drive, which cleared out the Temp folder and the downloaded/unpacked installer files (in C:\$Windows.~BT and C:\$Windows.~WS), then started the installer again and had no trouble.

I took the plunge and upgraded. Microsoft's Volume Activation Management Tool has this to say about the installations current state:

Until I actually try it, the ability to migrate the digital entitlement to new hardware is still difficult to determine. Sources indicate that this is allowed if the OS you upgraded from was retail:

• If you upgraded from a retail copy of Windows 7, Windows 8 or 8.1, the Windows 10 license carries the retail rights from which it was derived.
• If you upgraded from an OEM Windows 7, Windows 8 or 8.1 license, these are licenses that come preinstalled on a new computer from a manufacturer, and then your Windows 10 license maintains the OEM rights.

This affects the rights to what you can do with the license. (Surprisingly issue free compared to what I'd anticipated.) If it's retail, you can continue to make hardware modifications to your system such as changing the motherboard or move it to a different computer. For an OEM version, if you change the motherboard, automatically, your free upgrade will be invalidated; meaning, you will have to purchase a new full retail Windows 10 license.

In Microsoft's Windows 10 license terms, section 4.b.:

Stand-alone software. If you acquired the software as stand-alone software (and also if you upgraded from software you acquired as stand-alone software), you may transfer the software to another device that belongs to you. You may also transfer the software to a device owned by someone else if (i) you are the first licensed user of the software and (ii) the new user agrees to the terms of this agreement. You may use the backup copy we allow you to make or the media that the software came on to transfer the software. Every time you transfer the software to a new device, you must remove the software from the prior device. You may not transfer the software to share licenses between devices.

I think the key bit is the part that reads: "and also if you upgraded from software you acquired as stand-alone software".