Gender Bender (Permanent/Temporarily switched their gender, not 'disguised'/dressed up).

Gangstar rivalry drank a juice that turned him into a girl

Friends since middleschool (now best friend), mysterious girl made the guy drink a potion which turned him into a girl

Ate a candy which turned him into a girl (candy is cursed), only happens when sun set. In order to return to be a boy, he will need a fluid inside of him (as a girl) :P or both ways

BF/GF - girl not fully satisfied with her BF sex life and lied to him for years (saying he is good) and thus the BF found out. Strange lightning struck the building then the next morning they switched their souls.

Childhood friends, switch their gender every midnight.

Drinking a wine /alcohol turns him into a woman

In the past, the devil was a male. Now reincarnated he is born as a woman.

Bus accident - exchanged souls with a classmate.

Read a diary and turned himself and his family into their opposite gender