I'm too bad of a writer to be scared of giving my ideas away, but I'd explore a pupae stage, where (layman disclaimer, if it isn't abundantly clear from my incoming butchering of the science) the butterfly is basically melted down into a goop with no "brain part" or "feet part" of the goop existing until reformed, yet retain their memories. If you divided that goop up and imagined there was a gestation time reduction, before combining it for the final stage of formation, you'd have some "plausible" (loose definition here) venue for it.