The payments provided for in this chapter shall apply only to debt service incurred pursuant to the provisions of this chapter. No payments or credits shall be allowed for any past debt service payments nor for any debt service payments beyond the date of the final interest and principal payment of the original issue of such bonds. Nothing herein shall restrict any city or county or entities or authorities thereof from refunding or refinancing such bonds within the original maturity schedule to take advantage of savings arising from such refunding or refinancing.