Any contractor for energy from the project failing or refusing to execute a contract modifying its existing contract to conform to this subchapter shall continue to pay the rates and charges provided for in its existing contract, subject to such periodic readjustments as are therein provided, in all respects as if this subchapter had not been passed, and so far as necessary to support such existing contract all of the provisions of the Project Act [43 U.S.C. 617 et seq.] shall remain in effect, anything in this subchapter inconsistent therewith notwithstanding.