Well, according to the PS2 game Naruto Narutimett, you'd upgrade in this manner when you progress from rank D missions to rank S mission.

Academy Student ----> Genin ----> Chuunin ----> Special Jounin ----> Jounin ----> Anbu ----> Hokage



I didn't make this out myself... this is from the game. In fact, I don't really understand why it is so.