Mo. Rev. Stat. § 89.191: Municipalities in first class charter counties annexing territory with a prior zoning classification, zoning not affected.

Whenever any city, town or village, located in a county of the first class with a charter form of government annexes any unincorporated territory, the zoning classification of the annexed territory shall remain the same as it was prior to the annexation, unless the zoning classification is affirmatively changed through the regular rezoning procedures used by the annexing city, town or village.