that "with no additional activity" seems backdoor-ish though. If I said some things in a chat expecting privacy per the agreement, then they change the agreement, does that mean they can collect my data from that chat going forward or does it mean they can collect it retroactively?
Well you have to think about LLMs actually work. There is no "going forward". Every new token is generated based on the entire context window (chat history).
then you can retroactively implicitly opt in for processing, and that's a dark pattern if I've ever heard of one