My suggestion for this is to implement a machine: The transfer/poketransfer.
There are probably several options for this to work but I thought of the following:
1) Use duplicate pokemon as currency (you lose it/them after the procedure is complete) in order to teach a pokemon their old gen/game moves.
2) Use special items (it could be wattz, special tokens or any item really, just like tutors) as currency.
3) Use a different pokemon that knows the move you want your pokemon to learn (you still need to sacrifice it).
4) Use the player's experience.
Additionally there could be another tutor for those moves but I think the machine I'm proposing would be a more interesting/appealing way to accomplish this for both players and devs.