The problem is the point of view of the people. They don't know "Jon" or "the stone" are making the decision for them, so they'd see it as very much arbitrary.
The idea of making it hard to change back and forth without some degree of effort might help with the "changing willy-nilly" issue, IMHO. OR at least making people have to spend - say - three months in each form until they decide.
Basically, we may be disagreeing on a bit of principle here, so I'll clarify where I'm coming from. Keep in mind I'm NOT claiming I'm right, nor am I criticizing you in any way.
Your scenario in-story seems to value overall "long-run" happiness. And that's a valid point of view. Mine values overall freedom over said happiness. Happiness is decreased, yes, but people get to make their own decisions.
