(1) The retirement benefit payable to a member eligible for disability retirement for duty-related reasons and granted prior to July 1, 1977, is 50% of the member's highest average compensation.
(2) A retired member receiving a disability retirement benefit on July 1, 1977, who has previously been granted a duty-related disability under provisions in effect on June 30, 1977, is subject to the provisions of this section after July 1, 1977.