If a unified school district, after the effective date of this section, applies for and is granted an apportionment under this chapter on the basis of grade levels as defined in Section 16003, all unpaid balances of prior apportionments made to the district, subject to Section 16161, shall be added to the balances of the apportionments made on the basis of Section 16003. In those cases, no further computations or repayments under Sections 16070, 16072, 16074, 16075 and 16084 shall be made with respect to the prior apportionments alone, but the computations and repayments shall thenceforth be based solely upon the combined apportionments, and shall be made as provided in Sections 16071, 16072, 16074, 16075 and 16084.