What happens when the head of a bureaucracy resigns, retires, or dies? according to weber, in a bureaucracy, assignments flow and accountability flows

When a bureaucratic head leaves his/her post, someone previously beneath that person must be elevated to that job to preserve the bureaucratic structure. In a bureaucracy, assignments flow downward and accountability flows upward - in theory. However, in reality, the top levels of a bureaucracy often have the power and the motivation to redefine "accountability" when convenient.