If a city of the second class becomes a city of the first class, the mayor and city council shall divide the city into not less than three wards, as compact in form and equal in population as may be, the boundaries of which shall be defined by ordinance, to take effect at the next annual city election after reorganization except as provided in section 32-553. Each ward shall constitute an election district, except that when any ward has over five hundred legal voters, the mayor and city council may divide such ward into two or more election districts. If it is necessary to establish the staggering of terms by nominating and electing council members for terms of different durations at the same elections, the candidates receiving the greatest number of votes shall be nominated and have their names placed on the general election ballot.