(b) In applying the provisions of section eleven hundred nineteen with respect to pre-existing lump sum or unit price construction contracts to a tax on retail sales of tangible personal property or a compensating use tax imposed pursuant to the authority of section twelve hundred ten or twelve hundred eleven, all references in said section to the date of the enactment of article twenty-eight or the enactment of a law increasing the rate of tax imposed under said article shall be read as referring to the date of the enactment of the local law, ordinance or resolution imposing such local tax or increasing the rate thereof.