The board of supervisors of any county operating under the provisions of this chapter is hereby authorized to refund any bonded indebtedness of such county now outstanding, payable from the sources of revenue provided by this chapter, and to issue refunding bonds of such county, which shall be secured by and payable from the same sources of revenue as the bonds refunded thereby. The said refunding bonds may be issued by the board of supervisors of such county only after the question of the issue of said refunding bonds shall have been submitted to the qualified electors of such county at an election to be held for that purpose in said county, and only after the said issue has been authorized by a vote of three fifths of the qualified electors voting in an election to be ordered by the board of supervisors for that purpose, whenever any such board of supervisors shall find it necessary or advisable or in the best interest of such county so to do. Such refunding bonds may be issued only to redeem outstanding bonds at maturity, or on any date upon which outstanding bonds may be redeemable, or upon the voluntary surrender of outstanding bonds by the owners thereof. Such refunding bonds may be sold for not less than par and accrued interest, or may be exchanged for bonds to be refunded thereby; however, the issue of such refunding bonds shall be accomplished in such manner as to avoid payment by the county of interest on the refunding bonds and on the bonds refunded thereby for the same period of time.