All existing mineral leases, including oil and gas leases, which may have been issued or approved pursuant to section 5 of the Mineral Leasing Act for Acquired Lands of August 7, 1947 (61 Stat. 913, 915) [30 U.S.C. 354], or the Mineral Leasing Act of 1920 (41 Stat. 437) [30 U.S.C. 181 et seq.], as amended prior to October 17, 1975, shall remain in force and effect in accordance with the provisions