5 CFR § 837.804 - Finality of elections under this subpart.

Except as otherwise provided by this subpart, an election of coverage under, or annuity from, another retirement system, in lieu of CSRS or FERS coverage or annuity, or the election between simultaneous entitlements under CSRS or FERS, is final and conclusive for the period of simultaneous entitlement to coverage or annuity.