14 U.S.C. § 2502 — Retirement

(a) Every commissioned officer, warrant officer, or enlisted member who is retired under any provisions of this title shall be retired with the permanent grade or rate held at the time of retirement, unless entitled to retire with a higher grade or rate under any provision of this title or any other law. (b) Where an officer is entitled, under any provision of law, to retire with one grade higher than the grade in which serving at the time of retirement, the next higher grade in the case of captain shall be rear admiral (lower half), and the next higher grade in the case of commissioned warrant officer shall be lieutenant (junior grade).