The Commissioners Court shall have the power to issue bonds of the authority to refund bonds of the authority which have heretofore been voted and which are outstanding at the time of the adoption of the refunding bond order, and no election therefor shall be necessary. Such refunding bonds shall mature serially or otherwise in not to exceed forty (40) years from their date, and shall bear interest at the same or lower rate than the bonds to be refunded unless it is shown mathematically that a saving will result in the total amount of interest to be paid. Such refunding bonds shall be approved by the Attorney General of Texas, and shall be registered by the Comptroller of Public Accounts of Texas upon surrender and cancellation of the bonds to be refunded, but in lieu thereof, the order authorizing their issuance may provide that they shall be sold and the proceeds thereof deposited in the place or places where the bonds to be refunded are payable, in which case the refunding bonds may be issued in an amount sufficient to pay the principal of the bonds to be refunded and the interest on such bonds to their option or maturity date, and the Comptroller shall register them without the surrender and cancellation of the bonds to be refunded. All such refunding bonds, after they have been approved by the Attorney General and registered by the Comptroller, shall be incontestable for any cause except for forgery or fraud.