The boundary line of any of the said districts may be changed hereafter by ordinance passed by a four-fifths vote of the council: provided, that said districts shall be kept as nearly equal in population as possible; and provided further, that except where the city boundary is changed either by annexation or exclusion of territory, such change of district lines shall not be made oftener than once in two years nor within a period of ninety days prior to any general municipal election.