18+ Megu♥Milk

Just before they enter high school, the boys Meguru and Shou are whining about the fact that neither of them has a girlfriend. A strange, large-breasted woman shows up, claiming that she will fix their problem, and kisses Meguru. This odd occurrence is pushed out of the boys' minds when they meet up with Souko, a friend from elementary school, who is going to be in their high school. She's grown up into a pretty young girl, and it seems like the boys might have a shot with her!

That evening, Meguru realizes that the strange kiss may have had more meaning than he thought: he's turned into a girl! He convinces Shou and his mother of his identity, but no one has any idea how to turn him back. Now he's going to have to learn how to be a girl and let go of his hope of dating his cute childhood friend Souko. What will happen to his hopes for a popular high school life?

(Source: MangaHelpers)