The board of trustees, at any time, may cause any employee of the city who has been retired by reason of physical or mental disability to be brought before it and again examined by three competent physicians appointed by the board to discover whether such disability continues, whether it can be improved, and whether such retired employee should be continued on the pension roll, and the board may examine witnesses for such purposes. The question of continued disability or ability to perform regular or light duty shall be determined by the concurring report of at least two of the three examining physicians. Such employee shall have reasonable notice that the examination will be made and that the employee shall be present at the time of the taking of any testimony. The employee may examine the witnesses brought before the board and introduce evidence in the employee’s own behalf. All witnesses shall be examined under oath, which may be administered by any member of the board.