In order for any municipal corporation to proceed under this chapter, it shall file before June 1, 1963, with the Secretary of State and with the Director of the Legislative Reference Bureau a complete copy of its then existing charter. The copy shall show by appropriate citations the source of each section of the charter. The citations shall show the original source of each section of the charter and the source of all amendments thereto. The copy of the charter shall include a certificate from an attorney-at-law licensed to practice law in this State that the attorney has made a careful search of the local acts of the General Assembly relating to the municipal corporation and is of the opinion that the copy of the charter and the citations are true and correct. There shall also be a certificate of the chief executive officer attested with the seal of the municipal corporation that the chief executive officer verily believes the copy of the charter is true and correct. All subsequent amendments to the charter adopted by action of the General Assembly shall be filed within 20 days of becoming law.