A credential which was issued prior to July 1, 1973, shall remain in force as long as it is valid and continues to be valid under the law and regulations of the commission. The commission may not by regulation invalidate such valid credential unless it issues to the holder thereof, in substitution, a new credential authorized by other sections of this chapter which is no less restrictive than the credential for which it was substituted with respect to the kind of service which it authorizes, and the grades or classes of types of schools in which it authorizes services.