priority of liens.] It shall be lawful for any irrigation district, which has issued certificates of indebtedness for said local improvement as in this chapter provided, to issue in place thereof an amount of bonds not in excess of such issue of certificates, and sell the same, or any part thereof, or exchange the same, or any part thereof, with the holders of such previously issued certificates of indebtedness for the purpose of redeeming said certificates; provided, however, that all the provisions of this act and Chapter 20, Session Laws of 1919, regarding the authorization and issue of bonds shall apply; and, provided further, that the issuance of said bonds shall not release the lands of the local improvement district, or districts, from liability for special assessment for the payment thereof; and, provided further, that the lien of any issue of bonds of the district, or indebtedness arising from any contracts with the United States prior, in point of time, to the issue of bonds or certificates of indebtedness herein provided for, shall be deemed a prior lien.