Question

Can I trade my haunter to pokémon white 2 and then trade the gengar back and
If I can do this then how?

Comments

Trade to white 2 from which game?

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To pearl

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The Haunter will definitely not transfer back. https://pokemondb.net/pokebase/233293/ Though I have no source saying it won't evolve.

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https://pokemondb.net/pokebase/70206/would-transferring-pokemon-from-gba-cause-evolution-trade This is for transferring from GBA to DS, but I think that the same applies.

Answer 1

You cannot trade from White 2 to Pearl due to the fact that a legitimate method of transferring back through the generations is nonexistent with the sole exception of the Gen 1 and 2 Time Capsule. However, if you had another game in Gen 5 (Black, White, or Black 2) you could trade Haunter to one of those and trade back to get Gengar.