I don't know about the requirements; I did upgrade from 7 to 8 (while also replacing a hard drive) and then to 10 (because I wanted Bash for Windows). It works very well, but my computer is a desktop, and I did pay a bit for it back then. Meanwhile, a lot of computers had Win7 preinstalled, and I wouldn't call most of them powerful. Hence the caution - OEMs don't give a shit, they'll happily sell a laptop that can barely lift its own OS[0][1], so if you upgrade that kind of equipment you risk basically bricking it.

[0] - Just recently I've been "fixing" such one laptop for a friend of my mother - it was barely strong enough to lift its own Windows installation. I made it much more useful by just replacing the RAM die with a one I pried off a broken laptop that serves as an "organ donor". Hell, I found getting moar RAM to be a quick solution for most of the lower-end laptops that get "too slow".

[1] - this happens for mobile too (as I experienced first-hand), so my rule of thumb is that in terms of computers and smartphones, buying cheap is worse than not buying at all