10 U.S.C. § 12213

(a) Under such regulations as the Secretary of the Army may prescribe, and with the consent of the governor or other appropriate authority of the State concerned, an officer of the Army National Guard of the United States may be transferred in grade to the Army Reserve. (b) Unless discharged from his appointment as a Reserve, an officer of the Army National Guard of the United States whose Federal recognition as a member of the Army National Guard is withdrawn becomes a member of the Army Reserve. An officer who so becomes a member of the Army Reserve ceases to be a member of the Army National Guard of the United States.