In chatgpt, why can't i change response models after chat has started

yeah, I’m trying to figure out if I use the regular chat enough to justify 20 bucks a month.

Right now I have both playground token based account and the gptplus account

it’s on gptplus that gpt4 gets used up and you have to wait. but if you don’t want to wait, it will convert the whole thread to 3.5, but there is no turning back later.