>>55798151I think it depends on the pokemon. Otherwise ghosts would just outnumber everything. And your pokemon would never really die, just evolve again?Or devolve.
I guess the closest thing would be digivolution, when your Digimon dies it sometimes turns into an egg of a new digimon. That, but gastly.
Also some ghost pokemon can be ghosts. See ghost mimikyu from the anime.
I feel like some just swap bodies. Like Trevenent probably gets to big and possesses a new tree. Pumpkaboo die and posess a new pumpkin every pumpkin season I guess? This is just head canon.
