10 U.S.C. § 12106

(a) An enlisted member of the Army National Guard of the United States who ceases to be a member of the Army National Guard becomes a member of the Army Reserve unless he is also discharged from his enlistment as a Reserve. (b) An enlisted member of the Air National Guard of the United States who ceases to be a member of the Air National Guard becomes a member of the Air Force Reserve unless he is also discharged from his enlistment as a Reserve. (c) An enlisted member who becomes a member of the Army Reserve or the Air Force Reserve under this section ceases to be a member of the Army National Guard of the United States or the Air National Guard of the United States, as the case may be.