There I was, thought I'd got a decent way to get my tablet upgraded to 10 Pro from the truly awful basic that was on it.
So I got the "generic" 10 Pro key (free but you can't register with it), that allows you to clean install 10 and I used it to clean install my machine
Then I went to register it using a new key from my Windows 8.1 pro key set.
Which failed
I tried every key from Windows 7 Ultimate onwards.
They all failed too.
So my TechNet keys can't be used. But why did I even try? Well because I had to rebuild Mrs S' tablet which came with 8.1 Basic. After the install I used the "Add Features" process to give it my TechNet key. Which worked and I upgraded it to a fully capable Windows 10 Pro machine.
Right now I'm in the process of downloading a vanilla copy of Windows 8.1 (not that easy now and I didn't keep my TechNet 32 bit images as I don't install 32 bit normally). I've tried pretty much everything, including the installer for Mrs S' Tablet but that broke my digitiser software and I could not fix it.
Once that is done I will have the weary job of installing 8.1, getting the drives in, getting the digitizer up to date, registering with my Pro key and then upgrading to Windows 10 all over again.
If you don't know for absolutely sure that your Windows 7/8/8.1 key will work with Windows10, I suggest you don't play with trying to switch versions on Windows 10. Honestly it will NOT be worth the pain. For me it's just time and a bit of frustration and my Tablet it the #5 machine I use in a long list so is in no way vital.
On the flip side, I did find an interesting downloader in my searches to get some of the images downloaded that I've been using. Jdownloader2. It is still beta but interesting. You'll need to download the Java one (other option), Windows is not yet available...