Mo. Rev. Stat. § 50.1180: Law not to apply to certain political subdivisions.

Sections 50.1000 to 50.1300 shall not apply to counties of the first classification with a charter form of government or to a city not within a county. No employee in a county which accrues benefits pursuant to sections 50.1000 to 50.1300 shall lose any of those benefits accrued because the county where the employee serves or served subsequently adopts a charter or constitutional form of government and the county shall continue to assess and collect all fees and penalties provided pursuant to law to fund the county employees' retirement fund.