(1) The governing body of any unit is hereby authorized to provide by ordinance or resolution for the issuance of revenue refunding bonds of such unit for the purpose of refunding any revenue bonds then outstanding and which shall then have matured or are then redeemable or subject to redemption within 10 years thereafter or can be acquired for retirement and issued under the provisions of this part or any other law for the purpose of paying all or a part of the cost of a project as defined in this part. The governing body of any unit is further authorized to provide by ordinance or resolution for the issuance of revenue bonds of the unit for the combined purposes of:
(a) Paying the cost of any improvements of a project or of acquiring by purchase or of constructing an additional project or projects and of;
(b) Revenue refunding bonds of the unit which shall theretofore have been issued for such project and shall then be outstanding and which shall then have matured or are then redeemable or subject to redemption within 10 years thereafter or can be acquired for retirement.
The issuance of such revenue refunding bonds, the maturities and other details thereof, the rights of the holders thereof, and the duties of the governing body and of the unit in respect to the same, shall be governed by the foregoing provisions of this part insofar as the same may be applicable.
(2) If such outstanding revenue bonds to be refunded are not immediately redeemable the issuing unit shall have power to invest the proceeds of such revenue refunding bonds in direct obligations of the United States until the first date upon which such outstanding revenue bonds are redeemable prior to maturity, not in any event later than 10 years from the date of issuance of such revenue refunding bonds.