There was a limited time where you could update pre Win 10 systems to Win
10 without buying a new license. If you did that, Microsoft recorded this
fact in their central system.
That period is over.
No, it's not. Windows will no longer deliver the upgrade tool via automatic update, but you can download it here —
https://www.microsoft.com/en-us/software-download/windows10ISO — via the
Create Windows 10 Media Installation tool. It will definitely upgrade Windows 8 (as that's what would install on my laptop) and may do Windows 7 too.
This means I bought an unnecessary (by anyone's definition) Windows 10 Pro licence. It wasn't as cheap as Evan's copy (it's from a more-legit source), but it was still about $60. Quite a lot for one line of text ...
Stewart