If as a result of any proposed changes the legislative body shall determine that a supplemental assessment shall be necessary, the proceedings provided for by this chapter may either be combined with or conducted separately from the proceedings for such supplemental assessment. If said proposed changes shall eliminate or substantially reduce the estimated benefits to be received from the improvement by any subdivision of land within the assessment district, the legislative body shall not order any such changes without ordering a corresponding credit upon the individual assessment theretofore imposed upon such subdivision. Said credit shall be made in the manner provided in Section 10427.1.