10 U.S.C. § 626: Acceptance of promotions; oath of office

(a) An officer who is appointed to a higher grade under section 624 of this title is considered to have accepted such appointment on the date on which the appointment is made unless he expressly declines the appointment. (b) An officer who has served continuously since he subscribed to the oath of office prescribed in section 3331 of title 5 is not required to take a new oath upon appointment to a higher grade under section 624 of this title.