As they consider their positions as chosen by God, their positions as GB & FDS will carry on until death.
While they may train others as Hold Me - TM suggests, a trainee will not assume replacement of a GB member until that GB member passes on.
Previous head honchos ( WT Presidents) up until Fred Franz were presidents until death. During Franz's rein, power was removed from the President and was transferred to the GB so the President then became less significant and therefore replaceable without consequence.