Trying to upgrade win 7 pro, not accepting key

[dugimodo]

I got a second hand i5 cheap with a copy of win 7 pro on it. It has the COA and I checked with a keyfinder that the installed copy of windows matches the COA.
So I thought I'd upgrade to windows 10, but it's not cooperating with me, I know officially the offer has expired but I was of the understanding a fresh install with a windows 7 key still worked.

First off I used an official MS retail windows 10 home flash drive I previously purchased, it's a bit old now but I thought that might work to my advantage. It would not accept the key from the COA.
Next I skipped the key, manually chose win 10 pro and installed. When it got to the final stage and asked for a key I tried again and it still won't accept it. Neither attempt got as far as being online.
3rd attempt - carried on from attempt 2 but skipped the key and got windows up and running and online, then tried to activate and change the key - still no go

So right now I'm trying the last thing I could think off, wondering if my retail windows media is the reason the oem key is being rejected I am running an in place upgrade with the media creation tool directly on the machine in question from windows 7.
If this fails am I out of luck? I don't mind windows 7 at all and will stick with it if I need to. I just prefer to have all 3 machines on the same OS if possible.

[wainuitech]

Could possibly be the USB Flash Drive. Had something similar happen a while back, not accepting a Key.

Just did a fresh install with a W7 Home key last week, went fine. Did find that the latest version (1709) didn't activate but the previous 1703 did. (Some say they have not had any problems with 1709 activating)

I get the ISO's from https://tb.rg-adguard.net/public.php Under the dropdown select Windows (final).

[dugimodo]

I think it must have been the retail drive I used to install that was the issue. Using the media creation tool and upgrading directly from the website seems to have worked perfectly, and it only took about 20 mins (100Mb fibre & SSD). Presumably if I ever want to reinstall I'll need to download a fresh iso knowing that the flash drive media won't accept that key. Anyway sorry the question was a little premature, asked out of frustration.

I prefer a clean install but the upgrade seems to be running sweet so I'll leave it and see how I go.

Side note;
i5 3470, 4GB RAM, 500GB 2.5" WD blue, in win itx case & PSU, windows 7 pro - all for $150 + courier fees. I feel like I got a great deal.
I abandoned the case and psu as it's a proprietary PSU and the cooling fan is uncomfortably loud, but even so.