There's a lot of detail below but I'll ask my question at the top: How do I upgrade to Windows 10 from an installation of Windows 8 downgraded to Windows 7, assuming the product key is accepted by the Microsoft activation servers? I'm also finding a Windows 8 installation that won't boot and a Windows 7 installation that works fine but in which I cannot retrieve the product key, and if possible I'd like to know if this is normal.
Details:
I have inherited a user with a desktop workstation that is bound to a domain in a business environment. I have administrative access with a local account and with a domain account.
The computer is originally a Windows 8 system but it has been downgraded to Windows 7. I don't have experience with downgrades, nor do any of the professional contacts that I've reached out to. I want to upgrade this system to Windows 10. I don't have the product key (no media, no sticker) and I don't have useful information from the previous administrator.
I don't fully understand what I'm seeing. There is a Windows 7 installation on C: and a Windows 8 installation on D:. I don't know if this is normal for a downgrade. I cannot boot the Windows 8 installation. Attempts to do so show the Dell splash screen as I pass the BIOS, then a blank, black screen. Attempts to select Windows 8 to boot from safe mode result in a message that Windows cannot boot and I need to repair it with installation media (which I have not tried). CHKDSK finds and fixes problems with the installation but finds the same problems again on the next pass. The Windows 7 installation works perfectly. I don't know whether or not this is normal and I'm just not supposed to boot the original OS since it's been downgraded.
Although Microsoft officially states that free Windows upgrades to 10 have ended, I know that the Media Creation Tool has been working and resulting in valid licenses as recently as less than two weeks ago. When I run the tool from the Windows 7 installation it fails during setup stating that it has failed to validate the product key. I do not have a physical copy of the product key and when I attempt to retrieve it in CMD or Powershell I receive error messages indicating that it cannot be found. The same commands succeed on a separate computer.
Thank you.
EDIT 1: I found the product key using a third-party script named get_win8key: https://github.com/christian-korneck/get_win8key
At this point I need to manually pass this product key to the Windows 10 Update Assistant or the Windows Media Creation Tool, and I don't know how to do that. I'm trying to figure it out but guidance is welcome. I also need to know whether or not it's safe to run this from the downgraded Windows 7 installation.