a jedi knight becomes a master by appearing in 2 or more films..you could argue that obi never becomes a master but that could be because only he and yoda are left for a time..at any rate..they dont disappear if its not in the script!! by they way..Qui gon does make a cameo in episode II..when anakin snaps and kills the sand people you can clearly hear QG's voice say "ANAKIN NOO"