All appointments, including new appointments, employee reassignments to cover vacant positions, and head of department positions, shall be on a probationary basis for a period of six months from the date of appointment. For those positions in which the individual is required to meet state minimum standards, such as police officers and firefighters, the probationary period shall conclude upon fulfillment of the standards or the passage of six months, whichever is later. Before the expiration of the probationary period, the head of the department, with the consent of the mayor, may discharge a probationary employee. Regular status covered employees who are reassigned, including those reassigned to a head of department position, shall be returned either to their previous job or a similar job, if vacant, or placed on the layoff list.