Legacy moves are a very weird concept in my opinion. If the reason for the change was from a balance standpoint, why does it make sense that the 'mon would get to keep his over/under powered move? For instance in league of legends, if the champ you main gets nerfed, you dont get to play the pre nerf version just because it was your main. Everyone has to deal with the new version. Whether or not it's for balance there had to be some reason for the change, and it never makes any sense for you to keep the old move.

My opinion on what Niantic should do with legacy movesets is change all of the legacy movesets so they don't exist anymore. While this seems unfair to anyone who invested in a 'mon with a move being deleted, a fair way to compensate is for anyone with a 'mon that has a move being deleted allowed a one time choice of the move replacing it, with either the new move, or any of the other moves it can learn. This way, legacy movesets aren't a threat to balance or getting in the way of the standard, and people who invested in pokemon with these moves will be fairly compensated without just changing the move on them.

Tldr. Delete legacy movesets. Allow trainers to choose move that replaces the deleted move.