Good questions!
On The X Subscription, third-party models are only billed through top-ups, never from your included usage. If you don’t add credits, a third-party model won’t run and you’ll see a prompt to buy credits instead, so you won’t be charged by surprise. You can also turn off models you don’t want in Cursor Settings → Models so they don’t show up in the picker.
On your current Cursor plan, you can still change tiers from the billing page in your dashboard, so moving back up to Ultra should work the same way it did before.