The board of directors having adopted the plans of public works and apportionment of benefits method of financing, shall apportion the benefits thereof accruing to the several tracts of land within the district which will be benefited thereby, on a system of units. The land least benefited shall be apportioned one unit of assessment, and each tract receiving a greater benefit shall be apportioned a greater number of units or fraction thereof, according to the benefits received. Nothing contained herein shall prevent the district from establishing separate areas within the district so as to permit future allocation of costs for particular portions of the work to specific areas. This area method of allocation shall not be used in any district which has heretofore made a final apportionment of units of benefits and shall not thereafter be changed except by compliance with the procedure prescribed in sections 31-411 to 31-412.