5 CFR §847.431

1. (a)
   1. (1)
      1. (i)A NAFI employee who completed a qualifying move from FERS under § 847.402(a) may, instead of the election provided by § 847.411 (pertaining to elections of CSRS and FERS coverage), elect to remain subject to the current NAFI retirement system for all subsequent periods of service.
      2. (ii)Prior service under FERS becomes credible under the NAFI retirement system rules.
   2. (2)An NAFI employee who has had a previous opportunity to elect retirement coverage under § 847.202(e) or (f) is not excluded from making this election.
2. (b)A survivor may make an election under paragraph (a) of this section if the employee was otherwise eligible, but died before expiration of the time limit under § 847.304.
3. (c)An election under this section is irrevocable when received by the employing agency.