The "light gets brighter", that's what they say when there has been a change of doctrine.
The people who are Jehovah's Witnesses are trained to obey the "faithful and discreet slave". They believe the "faithful and discreet slave" is represented only by the Governing Body of Jehovah's Witnesses. The Governing Body at this time teaches that anyone who learns their way and stays close to it will be saved for a new life in a new (good) world. Any one who does NOT obey their way will be destroyed (by Almighty God).
The dilemna is such that the men to obey are human and they will die. When they do die, who will distribute the "new light" that is needed for salvation? Everyone, even the most faithful of the Jehovah's Witnesses will probably confess under pressure that there are mistakes in their doctrine which is called The Watchtower. So the problem is how will the mistakes be eliminated once the "faithful and discreet slave" is no longer among them?