For the purpose of refunding, extending or unifying the whole or any part of its valid bonded indebtedness, the district may issue refunding bonds not exceeding an amount of the principal of the outstanding indebtedness to be refunded and the accrued interest to the date of such refunding bonds. The board shall provide for the payment of interest and principal of such refunding bonds in the same manner as was provided for the payment of interest and principal of the bonds refunded.