Opening the locked door in the character swap room is really simple. Defeat bowser and then go around and collect all the whit glittering sparkley rabbits.they are in different locationswhwer rabbits already were and can only be caught by Mario.
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Some open locked doors.If it has a hat on it,you will unlock the character with that hat.
Catch all the glowing rabbits.
basically that the next big door is open and to go fight bowser the second time
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Is that the door that says "door is locked but seems unstable" (whatever) when tried opening it? If it is, just constantly press x on the door.. it'll eventually give in and open.....