Whenever a vacancy exists in any position in the service of the city, with the exception of common laborers, the vacancy shall be filled by appointment of one of the six persons who ranks highest on the appropriate eligible roster of the board or by transfer within the service of the city from another position of the same class. However, the ranking layoff of the same class shall be appointed in every instance. Whenever it is impossible for the board to certify eligible persons to a vacancy, the board may authorize the appointing authority to fill the vacancy temporarily pending the establishment of an eligible roster. No authorization may be given for longer than 120 days, and no employee shall acquire seniority or permanent status under this part by virtue of the temporary appointment. All appointments, other than temporary appointments, shall be probationary for 12 months from the date of appointment; provided, however, that any employee who obtained permanent status prior to the passage of the act adding this amendatory language shall maintain his or her permanent status. During any probationary period provided for under this act, the employee may be summarily dismissed, demoted, suspended, or otherwise disciplined by the appointing authority with or without cause and with no right of appeal to the board. After the expiration of the probationary period, an appointment shall become permanent.