Windows 10 was free for a year after launch for anyone who had an older version of Windows. For those who missed this transition period it was possible to get an upgrade right up until the end of 2017, a loophole Microsoft has now closed - although it wasn't much of a loophole, as the company knew all about it.
However there are other ways to upgrade to Windows 10 that don't involve getting the upgrade assistant from the official site.
It's unclear as to why this works, but if you have a product code for an old version of Windows 7, 8 or 8.1 you should be able to enter this into a copy of Windows 10 and get an activation. You will be given access to the version of Windows 10 that matches the original product key. So Windows 8 Pro will get Windows 10 Pro, while Windows 8 Home will get, you guessed it, Windows 10 Home.
To get Windows in this way, head to Microsoft's site and get the Media Creation Tool which allows you to either burn a DVD or make a USB boot device. You can also install over you existing build - but please do this only if you're sure you have backups of all your drives.
I noticed discussion of this at Windows expert Paul Thurrott's site, and have spent some of today setting up a Virtual Machine using Hyper-V to test it. And indeed, my old retail copy of Windows 7 Ultimate does indeed activate a new version of Windows 10. I had some problems with network connectivity - the ISO I downloaded from Microsoft seems to need an internet connection to work at all. But the product validates fine once it's online.
If you're interested in upgrading then this is probably worth a look. If you have an old laptop which has a product key on it somewhere - most do - then you may well be able to use that code to upgrade.
It's also worth pointing out that Windows 10 works different to previous versions of Windows. When you move to Windows 10 you get some account flags on your Microsoft account. I use Hotmail, for example, and my main desktop PC is flagged on that account as having a valid Windows 10 licence. As far as I can tell, Windows 10 is more open to upgrading parts - like, perhaps, the whole computer, and carrying on with your old Windows 10 licence.
As I pointed out in my article about the closing of the accessibility loophole, I don't think Microsoft really cares about end users getting free upgrades. It makes more money from OEM sales of Windows 10 on new laptops and revenue from corporate users than the slender pickings of home users. Indeed, Microsoft makes more money - and more margin - on selling cloud offerings these days.
Windows isn't a cash cow when it comes to home users, so I suspect there's a lot of give built into the system.
I want to be clear though - if you're trying this then you must have a backup of your system. It's a risky process, and while Windows 10 will allow you to roll back an upgrade I don't want you to lose precious files. So only undertake an upgrade or reinstall like this if you're confident you know what you're doing.