Not sure if this is it, but I will put it anyway.
According too Bulbapedia, after the main storyline...
The player starts out as their partner Pokémon for several days until
they complete the Worldcore and make a wish for their playable Pokémon
to return. The player decides whether to return or not and evolution
may not happen until they return to Pokémon Paradise. After the player
returns to Pokémon Paradise and completes one dungeon, both the player
and partner Pokémon may now evolve if the player chooses to evolve
them inside of dungeons. The leading Pokémon may also be changed at
any time in a dungeon as well.
I hope that's correct :)
Source: It's the 'bulbapedia' link up top...