Ed. Code, § 23400

(a) If, prior to attaining the age of 55 years, a person who is entitled to receive a retirement allowance because of his or her retirement for disability prior to July 1, 1972, engages in a gainful occupation, the board shall reduce the portion of his or her monthly retirement allowance that is not provided by his or her accumulated contributions, to an amount, which when added to the compensation earned monthly by him or her, does not exceed the amount of the compensation that would be earnable by a person holding the same or an equal position as or to that which he or she held at the time of his or her retirement, and in the same salary step or rating, or if no such position then exists, the compensation earnable immediately prior to its abolition.

(b) If his or her earnings are further altered, the board shall further alter that portion of his or her retirement allowance to the lower of the following amounts:

(1) The amount of that portion of his or her retirement allowance as it would be if not reduced under this section.

(2) An amount that, when added to the compensation earned by him or her, equals the amount of the compensation that would be earnable by a person holding the same or an equal position as or to that which he or she held at the time of his or her retirement, and in the same salary step or rating, or if no such position then exists, the compensation earnable immediately prior to its abolition.

(c) For purposes of this section, the retirement allowance subject to adjustment is the unmodified allowance irrespective of the option elected.

(d) When he or she attains the age of 55 years, his or her retirement allowance shall be made equal to the amount it would be if not reduced under this section, and may not again be modified under this section. Section 24015 does not apply to those persons affected by this section.