The provisions of Sections 57-1-1 through 57-1-51 shall not repeal or impair any law now in effect, except as therein specifically provided, but shall exist as a separate, several, independent, additional and cumulative method for giving to the people of Mississippi the fulfillment of the public policy of encouraging the promotion of economic development of new and existing "enterprises." Nor shall the aforesaid sections or any part thereof repeal any of the provisions of private or special municipal charters, nor affect, limit or restrict the right of any municipality, now operating under special charter, to amend said charter pursuant to the provisions of Section 21-17-9, Mississippi Code of 1972, which section shall apply to Sections 57-1-1 through 57-1-51.