the system shall pay the service retirement benefits for which the member or former member is eligible pursuant to part II, VII, or VIII of this chapter, as applicable, retroactive to the member's or former member's required beginning date with regular interest.
provided that if the system receives the written election after the member's or former member's required beginning date, but within sixty days following receipt by the member or former member of notice from the system that the member or former member is required to make the election, the written election shall apply, and the member's or former member's retirement benefit shall be recomputed, based on the written election, retroactive to the member or former member's required beginning date. The amount of any underpayment resulting from recomputing the benefit shall bear regular interest. If recomputing the benefit results in an overpayment, payments shall be adjusted so that the actuarial equivalent of the benefit to which the member or former member was correctly entitled shall be paid.
HRS § 88-74.7