Creation of the district does not affect any existing franchises granted by any unit of local government. Those existing franchises shall continue in full force and effect until legally terminated, and all ordinances and resolutions of the unit of local government regulating local natural gas systems shall continue in full force and effect unless superseded by rules of the district. This superseding, if any, may occur only on the basis of prior mutual agreement between the district and the respective unit of local government. (1997-426, s. 2.)