Mo. Rev. Stat. § 198.058

Any facility licensed under chapter 197 or chapter 198, which is in operation before September 28, 1979, or whose application is on file, or whose construction plans have been approved by the department before September 28, 1979, shall be exempt from construction standards developed by the department subsequent to the date such facility became first licensed and including those construction standards developed after September 28, 1979, for buildings or other physical units which were in existence or under construction on September 28, 1979. Such facilities shall be licensed in accordance with all other standards and regulations promulgated under sections 198.003 to 198.096.