(I'll just answer all your questions even though Kane1685 answered some incase it helps add clarity or something)
Hello!
I'd like to know if it is safe to transfer Pokémon downwards between generations. Now, entering in detail...
With "transfering downwards in generations", I mean, as an example, transfering from Generation 4 to Generation 3.
Now, here's what I mean by "safe", in this context:
- Does PKMN-Chest detect if the Pokémon being transfered to a past generation is a future generation one?
Yeah, it won't allow Pokemon that didn't exist in the generation (It'll just put it back in the chest, or leave ones that didn't exist when doing full box swap)
- Does PKMN-Chest warn me if the Pokémon or its moves don't exist in the past generation?
- In that case, does PKMN-Chest prevent non-existing Pokémon in that generation to be transfered? Does PKMN-Chest make the Pokémon forget the moves that don't exist in that generation (similar to how PKMN-Chest removes the Pokémon's HMs when transfering between generations)?
For Pokemon see above, for moves: It will remove all moves that don't exist in the generation you're transferring to. Currently it doesn't warn about that, I should probably add that though.
- Does PKMN-Chest mantain the Pokémon legal (or with its original stats) when transfering to past generations?
Perfect legality is not guaranteed, if something changed like it could have illegal moves if the moves existed in the older generation but that Pokemon wasn't allowed to learn them. The stats are kept the same though
- Does PKMN-Chest mantain the Pokémon legal (or with its original stats) when transfering it back to future generations?
Anything that didn't exist in the older generation can't be restored when transferring back up to the newer generation, most things should be fine though. I think forward transfers should work nearly identically to if you were to do the transfer in game.
- Insert any other questions about the program keeping legality
I think the most likely way for a Pokemon to be made illegal by just transferring is to transfer backwards and it to know a move that existed but couldn't learn in that generation (ex. Piplup can learn Covet from a tutor in B2W2, which exists in G4, but Piplup can't learn, thus a Piplup with Covet will keep in when transferred to G4 when it couldn't technically learn that)
That won't break anything in game, but isn't technically possible to have normally in G4.
Also, I haven't checked exactly how its handling this and what it causes to happen in game but it seems the met location and origin game don't get changed when transferring back so you can have a Pokemon from black 2 in platinum which is probably illegal, I don't think it causes issues though and does make transferring back forward keep the correct game, for location it just seems to get set to a seemingly random place (whichever place has the same ID I think)