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You cannot roll back your firmware (therefore if you subscribe in the future you will be subscribing at FSDS V12.3.x or later).
You can disable FSD by selecting the TACC (cruise control) or Autopilot (Beta) which gives you TACC plus lane keeping (but it is only for use on highways, not city streets.)
If your current firmware has changed any of the TACC or AP behaviours, you cannot go back to whatever behaviours you had before because firmware cannot be rolled back.
Welcome to owning a tesla, where the car you bought and learned to drive may not be the same tomorrow, for better or worse.
If your car is an early 2019 or newer model, it probably has basic AP including Autosteer, since they became standard around that time. I’m guessing that’s also a prerequisite for getting the FSD trial. Autosteer is automatic lane keeping.