The provision in Section 31681.8, when adopted or readopted, shall apply to members of this system or a superseded system who retired for disability, except that payments to a member of this system or a superseded system who retired for service-connected disability or to the surviving beneficiary of such member shall be computed on the basis that the member’s service had continued to age 60, if a safety member, or age 70, if any other category of member, provided, that this additional service credit shall not exceed 30 years.