First, go to the Map Screen. Press R, R, L, L, B, B, A, A, Select, Select, B. Note: Do this fast. If you did it correctly, you should hear Mario say "Mamma-mia! It's Bowser!" Then, turn off the Game Boy Advance. Turn it on, then when you go to the map screen, you will see Bowser dancing on the level you are on. Press Select to become a Bowser with a blue shell and B to go to the normal red shelled Bowser.