Willy/Milly (1986)

Milly always wondered what it was like to be a boy. This morning she woke up with her first clue

Milly is annoyed whenever her mom tells her to behave more appropriate for a girl. She's just not interested in fancy dresses or prom dances. Some day her best friend's little brother offers her a potion which can make her deepest wish come true during a solar eclipse. She goes through with it and ends up with male genitals, additionally. So she has to decide if she wants to life as boy or girl. Her father, who always wanted a son, supports her in checking out life as a boy. Hence Milly changes school and starts out as Willy.