14 U.S.C. § 2123

1. (a)An officer who receives an appointment under section 2121 of this title is considered to have accepted his appointment on its effective date, unless he expressly declines the appointment.
2. (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 his appointment in a higher grade.