Right. So they're a new species. As a general rule, at least in the Western World, they'd probably be given basic human rights by default ability to prove sapience (plus human ancestry- so if nothing else they'd be treated as deformed humans- and we don't generally treat people born with birth defects as nonhuman)... but they are *still a new species*, with core biological differences to humans...
Dietary needs is an excellent start. Foods like chocolate and caffeine- something of staples in the human diet- can be *fatal* to felines. And apparently grapes cause liver failure in dogs (no data on if it does the same to cats). They also don't do well with milk, garlic or tuna. Two of which, a lot of people who don't know anything about cats often feed to cats.
And other differences in biology. Sense of smell. Preference for smells humans dislike (musky, even moldy, scents). Distaste for things humans do like (citrus).
Then comes eyesight- which cats are significantly different from humans (they have vastly superior night vision, we have superior color vision and visual range).
Cats have muscles connected to their ears which let them twitch and point in the direction of sounds, in much the same way that humans will turn their heads toward sounds.
These are some of the many things that showcase the gap between human and nonhuman beyond "has fur and a tail", and could help show readers that this is a design decision with meaning.