First, please create an event with three pages and follow the instructions below.
You dont need exchange a specific pokémon :
Page 1 :

Page : 2

Page 3 :
You can put what you want on page 3


You need exchange specific Pokémon :
Page 1 :

Page 2 :
You can put what you want on page 2

commands used :

call_party_menu

$pokemon_party.has_pokemon?(25) == true

retirer_pokemon(25)

ajouter_pokemon(26)

$pokemon_party.actors.delete_at(gv[43])

Then, please save this script:

module GamePlay
  class Party_Menu
    CANCEL_VARIABLE = 675
    alias old_action_A action_A
    def action_A
      $game_variables[CANCEL_VARIABLE] = 2
      old_action_A
    end
    alias old_action_B action_B
    def action_B
      $game_variables[CANCEL_VARIABLE] = 0
      old_action_B
    end
  end
end