Wash. Rev. Code § 35A.06.070
By means of the procedures set forth in this chapter, insofar as they apply, any noncharter code city which has been governed under the provisions of this title for more than six years may abandon the classification of noncharter code city and elect to be governed under the general law relating to cities or towns of the classification held by such city immediately prior to becoming a noncharter code city, if any, or relating to cities or towns of the highest class for which it is qualified by population, with the powers conferred by law upon such class, while retaining the plan of government under which it is then organized. A change of classification approved by a majority of the voters voting on such proposition shall become effective upon the filing of the record of such election with the office of the secretary of state.
RCW 35A.06.070