If a youth switches from two handed to one handed, they have to start a new average. I think that part of the rule was there before the two two handed style of today. At the time it was originally written, it probably referred to the time when a much younger junior bowler no longer needed to throw the ball two handed and was able to step up to a more conventional release when they were a little older.
As for youths becoming pros that switch styles, very few pros switch styles. Sure they become versatile but using the same style changing hand positions, speed and so on. The only one I've seen that makes the drastic change, and it's mainly for certain spares, is Osku Palermaa. He does it to go hard and straight, much harder to do with the two handed style even with a plastic ball.
Ah, that theory makes sense. If the rule is as you say, it should be clarified. At the same time, for some people, switching from 2 hands to 1 hand might not be any more of a change than the usual changes that you mentioned. If those changes are accepted, why not accept occasional support from the non-dominant hand?