I'm with you on doing an upgrade rather than a clean install, I wanted the OS to be the only thing that changed. Desktop, bookmarks, files, etc... all still in place after the upgrade.
Hope you can work through it to get an upgrade rather than a clean install, my thoughts are to reverse what you've done, IMO uninstalling W10 should put you back to "OS is not W10 and this computer hasn't upgraded yet" status but I can't be sure about that.
I've upgraded a retail Win7 OS to W10 Home and it's activated, according to the EULA I can use that same activated OS to install on my system when I upgrade my MB and/or CPU and/or whatever other hardware needs replacing. Or if I build an entirely new system, how do I go about showing my W10 activated OS is no longer on a retired/replaced system.
Below is for the OP
I guess if you go back to the previous OS within 31 days your activated W10 would revert to your previous qualifying OS.
Apparently going back to your previous OS, or performing a clean install are the only things left to do.
You could therefore upgrade to 10, revert to 8.1 backup after 6 months, run 8.1 until end of life in 2018 and then upgrade or clean install 10 because your device is already registered on MS servers.
If you decide to use your SSD then Windows 8.1 will remain activated (forever) but if you (ever) upgrade it to 10 or clean install 10 that will be activated also as long as you are on the same device.
If you want to upgrade, the easiest is to copy the ISO to your desktop, mount it and click on set-up.exe. (Option 5 in this tutorial: Windows 10 - Upgrade Installation - Windows 10 Forums )
You can make an iso using media creation tool or download one. See here : Windows 10 ISO Download - Windows 10 Forums
Getting an ISO is better than upgrading direct as if something goes wrong you don't have to download the whole thing again.
See section 4, subsection b, in the EULA, this document below is from my current install of W10 Home. This addresses upgrading from "stand alone" software which I did. Might have to use the Win7 License that I upgraded from to activate an install of W10 in an upgraded, or rebuilt machine, at this point nobody can say for sure how this will work out. I do know I can't use the Win7 license I upgraded from to install Win7 in any computer without violating the W10 EULA while using the W10 upgrade software.
4. Transfer. The provisions of this section do not apply if you acquired the software as a consumer in Germany or in any of the countries listed on this site (aka.ms/transfer), in which case any transfer of the software to a third party, and the right to use it, must comply with applicable law.
a. Software preinstalled on device.
If you acquired the software preinstalled on a device (and also if you upgraded from software preinstalled on a device), you may transfer the license to use the software directly to another user, only with the licensed device. The transfer must include the software and, if provided with the device, an authentic Windows label including the product key. Before any permitted transfer, the other party must agree that this agreement applies to the transfer and use of the software.
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.