Pursuant to the provisions of the aforesaid amendment and this chapter, the corporations may, at any time and from time to time, issue for the state refunding bonds of the state for the purpose of refunding any or all of the bonds authorized by the aforesaid amendment then outstanding, including any refunding bonds that may have been previously issued, whether such refunding shall occur before, at or after the maturity of the bonds to be refunded. In the discretion of the corporations, refunding bonds may be issued in exchange for such outstanding bonds or they may be sold and the proceeds thereof applied to the purchase, redemption or payment of such outstanding bonds. Refunding bonds to be issued in exchange for such outstanding bonds shall not be issued in a principal amount greater than the principal amount of the bonds to be refunded. Refunding bonds to be sold pursuant hereto may be issued in such principal amount or amounts as shall be determined by said corporations. Pending the application of the proceeds of refunding bonds issued in accordance with this section, such proceeds, together with investment income therefrom, and moneys in any sinking fund for the bonds to be refunded, together with investment income therefrom, may be held by the State Treasurer, in trust, or may be deposited by the State Treasurer, in trust, on such terms as the Director of Finance shall approve, with one or more trustees or escrow agents which trustees or escrow agents shall be trust companies or national or state banks having powers of a trust company within or without the state, for investment in direct general obligations of, or obligations the payment of the principal of and interest on which are unconditionally and irrevocably guaranteed by, the United States of America. The proceeds of refunding bonds, together with the investment income therefrom, and moneys in any sinking fund for the bonds to be refunded, together with investment income therefrom, shall be available for the payment of all or any part of the principal, interest, and redemption premium, if any, of the bonds to be refunded and of such refunding bonds, or any of them, as the said corporation in its discretion shall prescribe. Proceeds of refunding bonds shall be so invested and applied as to assure that the principal, interest, and redemption premium, if any, on the bonds to be refunded thereby shall be paid in full on their respective maturity, interest, or redemption payment dates. The State Treasurer, with the approval of the Director of Finance, may contract with respect to the safekeeping and application of proceeds derived from the sale and issuance of refunding bonds and other funds included therewith and the income therefrom, including the right to appoint a trustee which may be any trust company or national or state bank having powers of a trust company within or without the state. As provided in the amendment, refunding bonds issued pursuant to the provisions of this chapter shall not be obligations of the corporations, but shall be general obligations of the State of Alabama, and the full faith and credit and taxing power of the state are hereby irrevocably pledged for the prompt and faithful payment of the principal of all refunding bonds and the interest and redemption premium, if any, thereon. Except as herein expressly provided otherwise, all provisions of this chapter regarding the terms and conditions of the bonds to be issued pursuant to Section 9-14A-2, as well as the sale, issuance, and execution thereof and the security therefor, shall apply to all refunding bonds issued hereunder provided, however, that no refunding bonds shall be issued unless the present value of all debt service on the refunding bonds, computed with a discount rate equal to the true interest rate of the refunding bonds and taking into account all underwriting discount and other issuance expenses, shall not be greater than 95 percent of the present value of all debt service on the bonds to be refunded, computed using the same discount rate and taking into account the underwriting discount and other issuance expenses originally applicable to such funds, determined as if such bonds to be refunded were paid and retired in accordance with the schedule of maturities, considering mandatory redemption as a scheduled maturity, provided at the time of their issuance. Provided further that the average maturity of the refunding bonds, as measured from the date of issuance of such refunding bonds, shall not exceed by more than three years the average maturity of the bonds to be refunded, as also measured from such date of issuance, with the average maturity of any principal amount of bonds to be determined by multiplying the principal of each maturity by the number of years, including any fractional part of a year, intervening between such date of issuance and each such maturity, taking the sum of all such products, and then dividing such sum by the aggregate principal amount of bonds for which the average maturity is to be determined.