Any variance or special use permit lawfully issued prior to the effective date of this chapter, or any amendment thereof, which could be lawfully issued pursuant to the provisions in effect after such effective date shall be deemed to be valid and continue valid after such effective date. Any structure or use lawfully authorized by any such variance or special use permit which could not be so issued after such effective date shall be allowed to continue subject to the provisions of this chapter dealing with legal nonconforming uses.