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The only "de-evolution" that has ever appeared in the games is Mega Evolutions reverting back to normal, (which are just battle-only forms, not actual species evolutions). For a Pokemon to be legal, you can only revert it as far back as the "untouched state it was originally obtainable in". If they distribute a Raichu as event Pokémon, it's already in that "untouched state", you can't make it into a Pikachu, because that pokemon "wasn't a Pichu/Pikachu in it's earliest legal state". You can only legally alter Pokémon into states that they could possibly be in if you were playing the game legitimately. There is nothing you can change in PKHeX to make that legal, because "legality" is determined by what's possible to do in the game. (If you figure out some secret ingame mechanic to de-evolve Pokemon in an unaltered retail copy of a game, please let us know :P)

Okay perfect! thank you for your help!