How to Update Probation Period for Existing Employees Using Position Synchronization?

We are trying to update the probation period for all existing employees. When we add probation data in Manage Positions and enable position synchronization (by checking the Probation Period checkbox), we run the scheduled process called Synchronize Person Assignment from Position Job. However, the probation period is not syncing for existing employees—it only updates for new employees. Could you please advise on how to update the probation period for existing employees?