If bonds are to be issued under this chapter for refunding of any bonds issued specifically to finance any electric power and energy project or facility and there are contracts in existence for the sale of electric power and energy generated by such project or facility wherein the cost of power to a purchaser specifically includes a portion of the debt service on the bonds to be refunded, such power contracts shall be amended to reflect in each year during the remaining terms of such contracts that portion of the savings to be realized from such refunding during each such year equal to the percentage of power output from such project or facility purchased by the purchaser under such power contracts. Nothing in this chapter shall be construed to alter, modify or change any such power contracts without the mutual agreement of the parties thereto.