The accrued liability contribution should be discontinued as soon as the accumulated reserve in the general reserve fund shall equal the present value as actuarially computed and approved by the board of trustees, of the total liability of the fund, less the present value computed on the basis of the normal contribution rate then in force of the prospective normal contributions to be received on account of persons who are at that time members. The accrued liability contribution rate may be decreased by the board of trustees provided the reduced level rate does not require an amortization period extending beyond the year 2010 for any accrued liability as of August 31, 1980. Subsequent to that date, any changes in the accrued liability by reason of changes in the benefits payable under the retirement system, changes in the actuarial assumptions, or changes in the actuarial funding method are to be amortized over a period not exceeding thirty years from the date in which the additional accrued liabilities are recognized in the annual actuarial valuation of the system.