Section 16. A city or town in which there is located an historic district established under a special law may, upon recommendation of the historic district commission having jurisdiction over such district, accept the provisions of this chapter with respect to such district by a two-thirds vote of the city council in a city or by two-thirds vote of a town meeting in a town, and thereafter such historic district shall be subject to the provisions of this chapter notwithstanding the terms of any special act pursuant to which such historic district was created. The provisions of this chapter shall not impair the validity of an historic district established under any special act.