Any drainage district or any sub-district, may for the purpose of readjusting, funding, refunding, extending or unifying any or all of its authorized indebtedness, bonded or otherwise, or both, may issue and sell funding or refunding bonds and with the proceeds of the sale thereof, pay the indebtedness, or may exchange the funding or refunding bonds with the holders of the outstanding indebtedness, in payment thereof in the manner hereinafter provided.