I'm not sure how this works in earler gen's either. Just never came up I guess. But say you breed a poke with another poke who knows an hm move, the baby inherits said move. When the baby wants to learn a new move can "said hm/egg move" be replaced by that move normally? Or do you have to go see a move deleter and do it like you would with any other Pokémon that's been taught an Hm?