Whenever any street, alley or public highway shall have been graded, or pavement shall have been constructed in conformity to grades established by authority of the city, and the expense thereof shall have been assessed upon lots or lands bounded by or abutting upon such street, alley or public highway, the owner or owners of such lots or lands shall not be subject to any special assessment occasioned by any subsequent change of grade in such pavement, street, alley or public highway, unless such change be asked for by a majority of the owners of such lots or lands; but the expense of all improvements occasioned by such change of grade shall be chargeable to and paid by the city.