After annexation is made the board may levy upon the annexed lands an assessment sufficient to equal the assessments for benefit originally paid by the lands of equal classification if the finding by the board as provided by section 468.119 was that the lands should have been included in the district when originally established, plus their proportionate share of the costs of any enlargement or extension of drains required to serve the annexed lands. If the finding of the board as provided in section 468.119 was based on the fact that additional lands are now benefited by virtue of the repair, improvement, or the change of the topographical conditions made to the district and were not benefited by the district as originally established, then the board shall levy upon the annexed lands an assessment sufficient to pay their proportionate share of the costs of the repair or improvement which was the basis for the lands being annexed. If the board finds that the lands are presently receiving benefits from the district but that some were reasonably omitted from the original establishment because of the change of the topographical conditions, the assessments levied upon the annexed lands shall be limited to a proportionate share of the costs of current and future maintenance, repairs and improvements.