Any entryman on an unpatented farm unit on a Federal irrigation project which shall be found by the Secretary of the Interior, pursuant to a land classification, to be insufficient to support a family shall be entitled, upon timely application to the Secretary to exchange his farm unit for another farm unit of unentered public land within the same or any other such project, or, upon terms and conditions satisfactory to the Secretary, for any other available farm unit on the same or any other such project. He shall be given credit under the homestead laws for residence, improvement, and cultivation made or performed upon the original entry, and if satisfactory final proof of residence, improvement, and cultivation has been made on the original entry it shall not be necessary to submit such proof upon the lieu entry. Rights under this subchapter shall not be assignable.