Pokémon Mystery Dungeon: Blue Rescue Team Tips

When you defeat Rayquaza at the end of the game, a hole near Wishcash's lake appears. Try to go into the cave and it says you can only go in the cave alone. Then, try to go to a dungeon and your partner will stop you. He says that we should go on some dungeons alone and choose different. After you say it's a good idea, your partner will leave to his friend area. Now you can go into the cave and evolve any Pokemon that can evolve. Just go to their friend area and choose "Make Leader". The Pokemon you were using will go to their friend area and you can use someone else.

Therefore, you can now also control only a single Pokemon.