Any town, city or borough, unless otherwise provided by special act, may by ordinance or by vote of its legislative body designate its zoning commission or its planning commission as the planning and zoning commission for such municipality, and such commission shall thereupon have all the powers and duties of both a planning commission and a zoning commission and shall supersede any previous planning commission or zoning commission, as the case may be. Such vote shall establish the number of members to comprise such planning and zoning commission, which number of members shall be five, six, seven, eight, nine or ten, not counting nonvoting members. In the establishment of a five-member planning and zoning commission, the provisions of section 8-19 shall apply. In the establishment of a planning and zoning commission with six or more members, the provisions of section 8-19 shall apply except that the terms of office shall be so arranged that not more than three of such terms on a six-member commission, four of such terms on a seven or an eight-member commission, or five of such terms on a nine or ten-member commission shall expire in any one year. Any public hearing conducted by a planning and zoning commission with six or more members shall be held by the commission or a committee thereof appointed for that purpose constituting a majority of the members of the commission. Any combined planning and zoning commission established under the general statutes prior to October 1, 1959, may continue to exist. Upon the establishment of a combined planning and zoning commission, all regulations adopted by the planning commission or the zoning commission which were in effect prior to the establishment of such combined commission shall continue in full force and effect until modified, repealed or superseded in accordance with the provisions of this chapter and chapter 126. A vacancy on such combined planning and zoning commission shall be filled in a manner prescribed by the legislative body of such municipality.