Dragon Ball Side Story: The Case of Being Reincarnated as Yamcha (Dragon Ball Gaiden: Tensei-shitara Yamcha Datta Ken) is a spin-off manga written and by Dragon Garow Lee, it is released in Shueisha's Shōnen Jump Plus digital manga magazine.
The manga begins with an ordinary boy who is a fan of Dragon Ball, he accidentally gets himself killed when he falls down stairs trying to get near a girl. Immediately afterwards, he finds himself reincarnated as Yamcha (during the Emperor Pilaf Saga), after getting his bearings Yamcha realizes that with his knowledge of events that are to come (such as his death by a Saibaman) he can change history, this leads to him going to train with Roshi immediately with Goku.
By the time of the Saiyan conflict Yamcha has managed to become much stronger than he had in the original story and so defeats the Saibamen by himself, even managing to survive multiple Saibamen grabbing onto him and Self Destruction. Prior to the Saiyan conflict, he goes to Namek and gets his potential unlocked by Grand Elder Guru, stating that knowing the story beforehand is truly advantageous.