Permanent means they don't feel the need to relook at the condition again, they feel confident they got it right. Non-permanent conditions are subject to regular reevaluations. Until it becomes protected they can change it, but something would need to happen to trigger them to relook at it. I.e. you get a surgery for a rated condition and it is successful or something.
20 years it the correct answer for a protected rating, only fraud can change those, even if it is clear the VA made a mistake.