So as an owner of ... quite a lot of Windows 7 Pro CD keys and completely legal / valid machines. I was surprised I was getting Windows 10 for free for myself to use, however I've just read the summary on this article, NOW it makes sense.
Key words in the summary are the following for the supported lifetime of that device.
So, unlike my Windows 7 key, where I can un-install it from the old PC / wipe the machine, then install it on a new machine, call a phone number, follow some prompts and re-activate my legit key, I get the impression the Windows 10 key I'm allocated will only work on ONE set of hardware, period.
I'm still not sure if they are simply going to convert my existing, legitimate Windows 7 key to a Windows 10 key or issue me a new key. Either way it sounds like a Win7 -> Win10 conversion license is is not going to be as flexible as a 'proper' Windows 10 license.
Also of note, if I recall the original articles regarding this, I have a 1 year window to make my Windows 7 keys "Win 10 free mode allowed" I'd bet good money, I can't just type the keys into a website and convert them, I bet I need to install Windows, using the Win7 key, activate it AND follow the prompts to engage "upgrade mode"
I'm seriously considering building a VM just before entering the key to actually do this.
Finally, I ALSO get the impression that a Windows 10 license / key is 'bound to' an email account / Micosoft account of some kind, which is also quite awkward for me, perhaps I can only have 1 Windows 10 license per account? Who knows.