(a) After district bonds have been authorized or issued or after a contract with the United States has been authorized or executed, if the board thinks it is necessary, it may authorize:
(1) modifications in the district and its improvements;
(2) purchase or construction of additional improvements and issuance of additional bonds based on the engineer’s report; or
(3) a supplemental contract with the United States.
(b) Before any of the projects under Subsection (a) of this section are undertaken, the board shall enter its findings in the minutes and shall give notice that an election will be held to approve the issuance of bonds or the execution of a contract with the United States. The election shall be held within the time and the returns made and the result determined in the same manner provided for the original bond election.