Mo. Rev. Stat. § 108.400

Any county of the first class having a charter form of government and containing the greater part of a city with a population of four hundred fifty thousand inhabitants or more having outstanding its general obligation bonds payable in whole or in part from ad valorem taxation, may from time to time, without an election, issue its general obligation refunding bonds pursuant to the provisions of sections 108.400 to 108.410 to refund any previous issue or part thereof of its outstanding general obligation bonds. The principal amount of the refunding bonds shall not exceed the principal amount of bonds being refunded, together with any matured and unpaid interest thereon to the date of the refunding.