303.53  Changes and amendments.
  The land use plan, once established, may be amended, supplemented, changed, modified, or repealed. In case, however, of a protest against a change signed by the owners of twenty percent or more either of the area included in the proposed change, or of the immediately adjacent area and within five hundred feet of the boundaries, the amendment shall not become effective except by the favorable vote of at least eighty percent of all of the members of the board of trustees.
83 Acts, ch 108, §13