Such regulations, restrictions and boundaries may, from time to time, be amended, supplemented, changed, modified or repealed. In case, however, of a protest against such change, signed by the owners of twenty (20) per cent. or more, either of the area of the lots included in such proposed change, or of those immediately adjacent in the rear, or adjoining on the sides thereof extending one hundred (100) feet therefrom, or of those directly opposite thereto extending one hundred (100) feet from the street frontage of such opposite lots, or when disapproved by the city planning commission, such amendment shall not become effective except by the favorable vote of three-fourths of all the members of the council of such city: Provided, That before any such amendment, supplement or change shall be voted on by the council, the city planning commission shall have been given twenty (20) days' notice thereof and of the date of hearing. The provisions of the previous section relative to public hearings and official notice shall apply equally to all changes or amendments.