No hospitalization of a mentally retarded person lawful before September 1, 1978, shall be deemed unlawful because of the enactment of this chapter. The board is authorized to establish reasonable regulations to require that the superintendent of each facility where mentally retarded persons are in residence apply under Code Section 37-4-42 for an order authorizing continued care of a client for whom such care is necessary and who was initially hospitalized under an order of a court prior to September 1, 1978. Such prior orders of hospitalization entered by the courts, unless superseded at an earlier date by an order under this chapter, or unless such prior orders expire under their own terms at an earlier date, shall remain valid until 12 months following September 1, 1978, after which all such orders shall be null and void and of no effect.