Permanent employees who are promoted to a higher classification shall undergo the probationary period prescribed for the higher classification but shall have the right to demote to their former classification in their former department if a vacancy in their former classification exists. 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 Civil Service Rule XVI. If no less senior position exists, then the employee shall be removed from County service.