AFAIK this has always worked. From what I recall, the official free upgrade period was one year after Windows 10 was released, which means it would have ended in 2016. Then Microsoft said that anyone using the assistive technologies would still be eligible. Well, that means you could you use features like magnifier or sticky keys and get the upgrade free. Also, that was on an honor system, no proof required. (I'm not sure how they would have proved it anyway!) I don't know what the official status is now, but I do know that it "just works".
Anyway, the upgrade has never not worked that I know of. I usually do a clean install and activate Windows 10 with a Windows 7 product key. One time I bought a new laptop that had Windows 10 Home. I had a Windows 7 Pro product key. I decided to see if I could just upgrade my laptop's W10 from Home to Pro. It worked! I just entered the W7 Pro product key through "Change product key or upgrade your edition of Windows", and my laptop's version instantly changed from Home to Pro. I did it after I had owned it awhile, and I didn't do a clean install. Nothing was lost.
It appears that Microsoft has loosened up activations quite a bit. For one thing, they want everyone on Windows 10. Also, maybe they need to compete with Apple who no longer charges for OS X version upgrades.