If the outstanding bonds issued for the acquisition or construction of the toll tunnel as originally designed are, at the time of issuance of the additional bonds, by their terms subject to immediate retirement before maturity at the option of the authority, or if the holders consent to the retirement of such bonds, the authority may include in the issue of bonds to be sold for the purpose of enlarging or extending the toll tunnel or changing the design thereof, an amount of bonds sufficient to produce funds with which to retire the outstanding bonds according to their terms. Before any such additional bonds are delivered the authority shall first duly call the outstanding bonds for redemption in accordance with their terms and conditions or obtain the consent of the holders thereof to their retirement.