Permanent employees who are promoted to a higher class shall undergo the probationary period prescribed for the higher class, but shall have the right to demote to their former class in their former department if rejected during their probationary period if a vacancy in their former class exists, except that trainees who are promoted to journey level classes shall not have the right to revert to their former trainee class if rejected during the probationary period.  If no vacancy exists, such employees shall be placed in the longest standing vacancy, as determined by the requisition form date, County‑wide.  If no vacancy exists, such employees shall displace the least senior employee as determined by Rule XVI.  If no less senior position exists, then the employee shall be removed from County service.