When you were using GPT-4o you likely uploaded files or did something that GPT-3.5 can’t handle which is why you can’t switch in that thread. If you start a new one, you should be able to access GPT-3.5 still.
If you’re getting that message, it likely means you’re getting a LOT more value than the $20/month you spend. The next step for a power user is Teams or to start looking at the API to build out your own app.