All appointments, including new appointments, employee reassignments to covered vacant positions, including 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 may, with the consent of the mayor, discharge the probationary employee. Regular status covered employees who were 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.