In connection with any repayment contract or other form of obligation, existing on August 4, 1939, to pay construction charges, providing for repayment on the basis of a definite period, the Secretary is authorized, upon request by the water users involved or their duly authorized representatives for amendment under this section of said contract or other form of obligation, and if in the Secretary's judgement such amendment is both practicable and in keeping with the general purpose of this subchapter, to amend said contract or other form of obligation so as to provide that the construction charges remaining unaccrued on the date of the amendment, or any later date agreed upon, shall be spread in definite annual installments on the basis of a longer definite period fixed in each case by the Secretary: Provided, That for any construction charges said longer period shall not exceed forty years, exclusive of 1931 and subsequent years to the extent of moratoria or deferments of construction charges due and payable for such years effected pursuant to Acts of Congress, from the date when the first installment of said construction charges become due and payable under the original obligation to pay said construction charges and in no event shall the unexpired part of said longer period exceed double the number of remaining years, as of the date of the amendment made pursuant to this subchapter, in which installments of said construction charges would become due and payable under said existing repayment contract or other form of obligation to pay construction charges.