The bonds of each series issued pursuant to this article may be issued as serial bonds payable in annual installments or as term bonds or as a combination thereof, and the principal of the bonds of each such series shall mature or be subject to mandatory redemption according to such schedule as the board of directors of said authority shall determine in the resolution authorizing the issuance of such series. Provided, however, no bonds shall be sold nor counsel or other professional hired or retained by the authority prior to February 1, 1999. Such bonds may be made subject to redemption prior to their respective maturities, at the option of the state, on such terms and conditions as shall be provided by the board of directors of said authority in the resolution authorizing the issuance of such series. Any or all of such bonds subject to redemption at the option of the state may be called for redemption by the authority pursuant to a resolution adopted by the board of directors thereof if pursuant to appropriations theretofore made by the Legislature, the moneys required for such redemption are at the time held in the State Treasury or if such redemption is to be effected with moneys provided by the sale and issuance of refunding bonds issued pursuant to Amendment 618 and as provided for in this article. The authority may specify the terms and conditions under which any of the bonds authorized pursuant to Amendment 618 may be exchanged for like bonds of other denominations as the authority may prescribe.