A district may also refund from time to time, whether at or prior to maturity, any outstanding indebtedness evidenced by its bonds, and may issue, and sell or exchange, bonds so to refund such indebtedness. Refunding bonds which are to be sold may be issued and sold at such time in advance of the time at which the bonds to be redeemed or paid out of the proceeds of such refunding bonds are to be so redeemed or paid as the board may determine.