Change various parts of characters
Select a character, but instead of pressing A again, press X, Y, or Start and it will show the character (it will not show the changes). Now press A again to start the match. Note: this changes mainly face related things, like hair style, hats, masks, etc. Not all characters have three different things, or necessarily any changes at all.
Level Effects Change
Just like costumes, presssing the X or Y at the level select screen will change effect of the level. For example it will turn off explosive ropes on the Wrestling Ring.