Will Dittos imposter ability automatically make it transform every time a new poke is sent out?

Question

or do you have to switch him out and then back in to use transform again?

Answer 1

No, Ditto's Imposter ability will only work upon switching in on a battle.

Trial: http://replay.pokemonshowdown.com/gen7ou-584058326

Ditto transforms back once inside its Pokeball.

Trial: http://replay.pokemonshowdown.com/gen7ou-584059239