After the publication of the official results of each United States decennial census, the board shall make such changes in district boundaries as are required to meet the tests of equitable population distribution among the districts with a minimum disruption of the then existing district pattern of service. In 1971 and every ten years thereafter, and also whenever the boundaries of such districts are changed, the board shall reapportion the representation in the governing body among the districts, and may, within the limits established in § 15.2-855, increase or decrease the number of districts. Each such reapportionment, other than decennial, shall become effective on January 1 following the year in which it occurs. If such reapportionment, other than decennial, results in the creation of a district or districts in which no member of the governing body resides, such vacancy shall be filled in the manner provided for by § 15.2-802. Each decennial reapportionment shall become effective as provided in § 24.2-311.