Whenever it shall have been duly ascertained by any census of enumeration taken under any law of the United States or of the state of Kansas or by any city that any city has attained a population of more than fifteen thousand (15,000), such fact shall thereupon, by the governing body of such city, be certified to the governor of the state, who shall thereupon by public proclamation declare such city to be a city of the first class. If the governing body of any city which has attained a population of more than fifteen thousand (15,000) and less than twenty-five thousand (25,000) shall determine by resolution duly adopted that it would be more advantageous for such city to continue to operate as a city of the second class, such governing body shall not be required to so certify the population of such city to the governor and the laws relating to cities of the second class shall continue to be applicable to such city.

The change in classification of any city under the provisions of this section shall take effect on the date the proclamation of the governor is issued unless a later date is specified therein, in which case the change and classification shall take place on such later date. The governing body of any such city, holding office at the time of the proclamation, shall continue to be the governing body of such city and the members of such body shall hold their respective offices until the next city election in an odd-numbered year.