Mo. Rev. Stat. § 48.053

The incumbent of the office of county treasurer of a county changing from third classification to second or first classification or from second classification to third classification shall continue to hold office for the term to which he or she was elected. The successor in office shall be elected at the general election next preceding the expiration of the incumbent's term of office to a term of two years and until a successor is elected, as provided in section 54.010, and qualified.