Cities are hereby divided, for legislative purposes, into classes as follows:
First class -- Those having more than 100,000 inhabitants provided that once a city is defined to be of the first class, it shall not be reclassified unless its population decreases by 25 percent from the census figures which last qualified the city for inclusion in the class;
Second class -- Those having more than 20,000 and not more than 100,000 inhabitants;
Third class -- Those having more than 10,000 and not more than 20,000 inhabitants, and
Fourth class -- Those having not more than 10,000 inhabitants.
Changes in classification resulting from any future national census shall take effect upon the filing of certified copies of the census in the office of the secretary of state as provided in section 600.18. Meanwhile the council or other governing body shall take measures for the election of proper officials and for dividing the city into wards, if necessary, and otherwise prepare for the coming change.