; and (iii) has declined to enroll in the supplementary medical insurance program under part B of title XVIII of the Social Security Act ( 42 U.S.C. 1395j et seq.). . (2) Allowance of one change of enrollment Such section is further amended by adding at the end the following new paragraph: (6) (A) Except as provided in subparagraph (B), after the end of the special enrollment period provided under section 2(a)(3) of the Fair Access to Insurance for Retired (FAIR) Heroes Act of 2017 , an individual described in paragraph (2)(B) may switch only once from enrollment under the Medicare program under title XVIII of the Social Security Act ( 42 U.S.C. 1395 et seq.) to enrollment in a plan contracted for under subsection (a). (B) The limitation under subparagraph (A) does not apply to enrollment by an individual in a plan contracted for under subsection (a) by reason of termination of the entitlement of the individual to a benefit described in subparagraph (A) of section 226(b)(2) of the Social Security Act ( 42 U.S.C. 426(b)(2) ) due to the performance of substantial gainful activity. . (3) Special enrollment period (A) In general The Secretary of Defense shall provide for a special enrollment period during which an individual described in subsection (d)(2)(B) of section 1086 of title 10, United States Code, may enroll in a health care plan under such section. Such period shall begin as soon as possible after the date of the enactment of this Act and shall end 12 months later. (B) Coverage period In the case of an individual who enrolls during the special enrollment period provided under subparagraph (A), the coverage period under section 1086 of title 10, United States Code, shall begin on the first day of the month following the month in which the individual enrolls. (4) Conforming amendments Section 1086(d) of title 10, United States Code, is amended— (A) in paragraph (4)(A), in the matter preceding clause (i), by striking paragraph (2)(B) and inserting paragraph (2)(A)(ii)