Following a promotion, an employee shall serve a one year probationary period in the new position which is considered part of the selection process. During the probationary period, the appointing officer may return a probationer to the probationer's previous rank by filing with the commission a statement in writing of such action, together with the reasons therefor as to such appointing officer may deem convincing and such return to previous rank shall become effective at once unless disapproved by the commission.