The liability for the increase in benefits provided by sections 7-439b to 7-439d, inclusive, for retirement allowances based on service rendered before July 1, 1979, shall be discharged by extending the period required for the annual amortization payments being made by the municipality under § 7-441 before July 1, 1977, until the date when the total past service liability shall be discharged. Such date shall not be subject to the limits provided in subsection (b) of § 7-441. The proportion of contributions paid to the Retirement Commission monthly under the terms of subsection (c) of said section shall, effective July 1, 1979, include the cost of applying the adjustments of sections 7-439b to 7-439d, inclusive, to retirement allowances credited for service rendered after July 1, 1979.