The work on said improvement shall begin at a point or at points to be determined by said commissioners, and said work shall be completed with all expedition possible, and said board of commissioners of such drainage district, or any contractor thereunder, shall have no power whatever to change such route or system of improvements or the manner of doing the work therein so as to make any radical changes in said improvements, without the written consent of all of the landowners to be benefited thereby, and of the landowners who may be damaged thereby.