Dragon Ball Gaiden: Tensei shitara Yamcha Datta Ken

A Japanese high school student is on his way to a Dragon Ball event with a friend. Suddenly, he sees a cute girl wearing a short skirt, but when he runs over to get a closer look, he trips and gets knocked out. Upon waking up, he realizes he's turned into Yamcha from Dragon Ball. Being an avid fan of the series, he knows what lies ahead in Yamcha's future, thus he decides to take matters into his own hands, in order to change Yamcha's fate. (Source: MU)