Section 3. Every city having a population of less than one hundred and fifty thousand and every town may, in the manner provided in this chapter, adopt any of the optional forms of municipal administration which are provided in this chapter and may, in the manner provided in section nine provide for the revocation or rescission of any such acceptance. The adoption of any plan under the provisions of this chapter shall be deemed to be an amendment to any contrary provisions of a local charter or laws having the force of a charter.