Why are people still asking about having to use their W7/8 key to upgrade or clean installs?
1 - It really is quite simple, if you haven't got W10 already then download from here
W10 Download - you don't need to input any key, W10 will automatically find the key if you older windowws copy is genuine.
2 - If you want to do a clean install you HAVE to follow above and then go to recovery/refresh and reinstall it from there. Again you don't need your key, just skip past the key activation a couple of times and when it gets to desktop it will have been activated from your previous key.
3- The key is then affiliated with your hardware, most likely mobo only and if you upgrade your mobo then you have until 29th July 2016 to reuse your previous W7/8 key in order to follow steps 1 and 2 again though you will probably have to call the MS activation phone number but that process is straight forward.
4 - Point 3 becomes invalid after 29/07/16 and as far as I can see you will have to pay for W10 on new builds if using a new motherboard. In one of my earlier posts I said this was likely the business model MS were using and the internet seems to be confirming this. Naughty on MS part but they still want our money and clearly had the plan to do so from the off.
If anyone can add any more value to this please do but it's more a case of people not reading through the last 4/5 pages of the thread