And the court may, on the petition of the commissioners authorize them to refund any lawful indebtedness of the district by taking up and cancelling all of its outstanding notes and bonds, as fast as they become due, or before, if the holders thereof will surrender the same, and issuing in lieu thereof new notes, or bonds of such district, payable in such longer time as the court shall deem proper, not to exceed in the aggregate the amount of all notes and bonds of the district then outstanding, and the unpaid, accrued interest thereon, and bearing interest not exceeding eight percent per annum.