Any judge who has retired or who retires as provided in Kan. Stat. Ann. § 20-2608, and amendments thereto, and who has entered into agreement with the Kansas supreme court to perform assigned judicial duties as provided pursuant to Kan. Stat. Ann. § 20-2622, and amendments thereto, may elect to purchase, subject to the provisions of Kan. Stat. Ann. § 74-49,123, service credit for such service under the retirement system for judges. Such purchase shall be by means of a single lump-sum payment. Such lump-sum payment shall be an amount determined by the actuary using the judge’s current age, the final average salary of such judge at the time of such judge’s retirement, the form of the payment of the annuity to such judge including any retirement option elected by such judge pursuant to Kan. Stat. Ann. § 20-2610a, and amendments thereto, and the actuarial assumptions and tables then in use by the system.