The council or other legislative body of any city or town in which there has been established or may hereafter be established by ordinance or pursuant to authority granted, or hereafter granted, by any of the laws of the state of Washington, any retirement system, pension, relief or disability system, excluding any system directly established by the legislature of the state of Washington and by its terms made compulsory, shall have the right by a legal contract in writing to merge or integrate its existing system with that of the statewide city employees' retirement system established by chapter 71, Laws of 1947 [chapter 41.44 RCW]. Any such contract shall contain appropriate provisions granting to any member of the systems integrated or merged the right to elect to withdraw his or her accumulated contributions accrued to the effective date of the merger or integration where the contract would result in a reduction or impairment of the benefits provided for in the existing system of which he or she is a member, and no such contracts shall be effective which shall reduce or impair the benefits which employees who are receiving benefits from either of the integrated systems would have received had the integration or merger not been effected.