Whenever an assessment has been levied by the board upon land in the district for general administrative expenses and other expenses not pertaining to any particular project, and the boundaries of the district have been or shall be extended so as to include land other than the land included within the district at the time the assessment was levied, the board shall make an estimate of the fair and equitable amount which should be contributed by the land included by the change of boundaries for the purposes of the assessment previously levied.