We have multiple anecdotes indicating people have had problems; we have at least one indicating people don't. This is, to me, clearly evidence against both "Everyone has problems" and "Nobody has problems" and in favor of "Some people have problems", and for the specific policy question at hand, we're trying to evaluate "Some people have problems".
I'm not sure how long that would take and if it would happen with all people but just like it takes a while before newborns can digest food properly I suspect that a similar adaptation would not happen overnight in adults that radically change their diet from one set of foods to another.
Apropos: vegetable matter is far harder to digest in general than meat (due to plants having cell walls that need to be broken down first).
Whether there's any particular guarantee of this happening is another question entirely and completely irrelevant. It would be relevant if you could determine that there is a guarantee this won't happen, but that's it.