N.J. Stat. Ann. § 40A:6-7

When any city becomes, by reason of its population, a member of another class in accordance with the classification above referred to, the laws before that time applicable to the city shall cease to have any effect, and thereafter the city shall be governed only by the laws applicable to the classification in which it is contained; provided, however, that nothing contained in this article shall affect or make void any contracts or rights or any act theretofore done by such city; or in any manner affect or change the internal structure or arrangement thereof or the laws affecting the same during a reasonable time until the newly applicable laws are adopted by ordinance of the governing body of said city.