261.5.6. If a significant alteration, within the meaning of the second paragraph, is made to the property assessment roll retroactively to a date included in a fiscal year preceding the current fiscal year, if that alteration results in a supplement to be paid or an overpayment to be refunded in respect of an amount of a tax, compensation or mode of tariffing referred to in section 261.5.3 and imposed for that preceding fiscal year, and if that supplement or overpayment has an effect on the revenues of the municipality for the current fiscal year, that effect is not taken into consideration for the purpose of establishing the aggregate taxation rate for the current fiscal year.
An alteration which raises or lowers the taxable value of a unit of assessment is considered significant if it has the effect of raising or lowering the total of the taxable values entered on the property assessment roll by more than 1%. For the purposes of this paragraph, that total is the one entered in the summary of the roll produced during the last half of the fiscal year preceding the current fiscal year in accordance with the regulation made under paragraph 1 of section 263.