The Secretary of Agriculture is authorized to amend any lease entered into with respect to lands under the jurisdiction of the Forest Service providing for the construction, maintenance, and operation of commercial recreational facilities at a Federal reservoir project so as to provide for the adjustment, either by increase or decrease, from time to time during the term of such lease of the amount of rental or other consideration payable to the United States under such lease, when and to the extent he determines such adjustment to be necessary or advisable in the public interest. No adjustment shall be made under the authority of this section so as to increase or decrease the amount of rental or other consideration payable under such lease for any period prior to the date of such adjustment.