If two or more districts have been included in one contract between the Board of Administration of the Public Employees’ Retirement System, and the governing board of the districts, as provided in paragraph (7) of subdivision (b) of Section 24810, and if, since that inclusion, two or more of the districts have been combined into a unified district, the unified district, by amendment to the contract, may, notwithstanding Section 20580 of the Government Code, be substituted for the districts so combined, and for all purposes of the contract.