A new charter shall entirely supersede the prior charter of a city. All ordinances and administrative acts or rules theretofore adopted by the governing body or administrative agencies of a city which are in conflict with or are inconsistent with a new charter shall continue in force for sixty days after the effective date of the new charter, unless sooner modified or repealed by competent authority; but at the end of this period shall, to the extent of such conflict or inconsistency, be of no further force or effect.