Section 49. If, when bonds payable from the sinking fund become due, the sinking fund commissioners of a city hold any other bonds of the city not due and payable, which have been stamped as the property of said sinking fund and are not negotiable, they shall so certify to the mayor and request that new bonds be issued to them by the city in lieu of the bonds not due. If the mayor is satisfied that there is occasion therefor, he shall cause new bonds to be prepared, bearing the same rate of interest, and payable as to principal and interest on the same dates as the bonds to be surrendered, and identical in terms therewith; and when executed by him and the city treasurer and approved by the committee on finance of the city council, they shall be issued to such commissioners on the surrender by them to the city treasurer and the cancellation of such former bonds. Such cancellation and new issue shall be entered in full on the books of the city treasurer.