Where any slough, marsh, or lake has been drained into the drainage ditches of any drainage district which has completed its work of construction, lands benefited by the draining of the slough, marsh, or lake may be added to the drainage district in the manner provided by § 14-121-703, but in no case shall lands paying taxes in another drainage district be added to any drainage district under the provisions of this subchapter, and in no case shall lands, lots, or blocks already in a drainage district be in any way affected by the provisions of this subchapter.