Or power creep, whichever you prefer. Some got entirely new powers, sometimes the powers they already had changed, and sometimes they lost some powers. I'm only referring to lasting changes here, not one-offs or new powers that quickly disappeared.
So, what you some that you think were good and bad ideas?
Superman is the classic case. I'm so mired in his later powersets, that I can only sort of look at the older, lesser ones in an almost academic sense. I was introduced to Superman through movies, cartoons, and general pop culture from birth, so it is to some degree, just what he's supposed to be. Besides that, though, I can say there are certain types of stories that his higher power level enabled, and other types of stories - or at least actions - that didn't work anymore after he got powerful enough.
Raven is another one who got more powers. While I watched the Teen Titans cartoon, I've mostly forgotten it, and was reintroduced to the character through reading the 1980s series, and I think she works better with powers she had when I was introduced to her. You'll see that theme a lot from me. But I don't think I'd call her increased powers anywhere near the worst.
Kon - now there's one where I'll say his increased powers were a severe detriment. To me they are part and parcel of making him a Clark clone (rather literally, at least 50%), which I dislike a lot. But also his original power of TTK was a lot more unique and fun to read as something different and it enabled some vulnerabilities, which I think is a good idea.
I'll go opposite on Fire, and pick after Invasion. Perhaps because I haven't read much of Bea before the changeup. I admit, my preference on this is mostly because I like the visual. My feelings are not strong on the matter, though.
Very much against speed thief, electrokinesis , and scouts. They're already really, super powerful because of their speed, and don't need more additions.