Go to mega's menu and press select then you'll see a blank screen press the correct blank spots which is coded in letters below. The ones I'm gonna do are the form codes. After you transform into one of them you automatically turn into that form you used at the start of the battle but if you want to turn back into normal MegaMan you have to reset your game.

[A] [E] [I] [M] [Q]
[B] [F] [J] [N] [R]
[C] [G] [K] [O] [S]
[D] [H] [L] [P] [T]

Effect	Code
Become Ninja at start of battle	QRICAE
Become Rouge with many boosts at start of battle	SLNDPC
Become Saurian at start of battle	IMFCGO
Become Tribe King at start of battle	ICGOSL
Become Zerker at start at start of battle	NIFCDG