I don't think they can change the TOS and use it against you if you didn't agree to that version. The whole thing about adoptables is that I don't think these are legally binding? I'm not even sure if you can "own" a design (unless it's trademarked and all that). However, they could possibly request that you don't use their original art anymore, since usually the rights to the art are kept by the artist.

I saw another artist that once said something like "I could just take back the design if I wanted to, but I would never do that!" about a design someone else ownedhad bought from them, that was a big red flag for me. They did end up doing that later even after saying they wouldn't ever do it.