Mo. Rev. Stat. § 115.168: Party affiliation change, procedure.

1. If a registered voter chooses to change his or her political party affiliation, the voter may notify the election authority of such change. Any change of political party affiliation shall be made by signed, written notice in substantially the same manner as a change of address application is filed under section 115.165.

2. For purposes of this section, the phrase "change his or her political party affiliation" shall mean changing affiliation from one established political party to another established political party, changing from affiliation with an established political party to unaffiliated, or changing from unaffiliated to affiliation with an established political party.