I've heard you can activate Windows 10 with a key from 7 or 8.1 [1].

[1] https://arstechnica.com/information-technology/2017/05/windo...

As graton alludes to with his upgrade link, the trick is to get to the upgrade through an accessibility link. Windows 10 is still free for people that require accessibility services (presumably since it has better accessibility features), but there's no actual requirement behind that.

You can still activate W10 using 7/8 keys and most of the other methods still work. At least that's what people say on /r/windows10.

I really like most of windows 10. My main complaint is the start menu, I have yet to figure out how I'm supposed to use it

I downloaded the iso during the free upgrade period but didn't get around to installing it until last weekend.

On my first attempt I agreed to a prompt to download updates to the installer; it then told me I needed to have a key to continue.

So I started again without downloading the updates, and I could install and activate without a key.