It's a good idea, but doing this would be hell. It's very very hard to replace sprites in this game.
From what I see, you'd need to:
-> Remove one of the characters idea, since the game's character limit is 5(Unless someone does a fancy code)(I assume you're going to remove wario or waluigi)
->Recolor mario's sprite so it's luigi(the easiest...)
-> For the others(Either wario or waluigi), you'll need to draw it from scratch and inject it in the game with lazyshell... But here is the catch: Sprite are divided in a lot of mini sprites, which some are there when you walk, some are not there when you jump, etc. It's madness and I don't recommend someone to do this.