5 C.F.R. § 839.603
Can I later change my election?
Your election is irrevocable once your employer or OPM processes it. If you do not make a timely election, the resulting coverage (see § 839.602) is also irrevocable.
Your election is irrevocable once your employer or OPM processes it. If you do not make a timely election, the resulting coverage (see § 839.602) is also irrevocable.