This 1967 amendatory act shall take effect on January 1, 1968. Nothing in this 1967 amendatory act shall be construed to affect the validity of any agreement or contractual relationship entered into prior to such date, except that the rate of any service charge computed periodically on the outstanding balance in excess of that allowed by this 1967 amendatory act shall be reduced to a permissible rate on or before January 1, 1968.