5 CFR §1600.14

1. (a)If a uniformed service member elects to be covered by BRS, the member may make a contribution election at any time.
2. (b)[Reserved]
3. (c)If the member had elected to make TSP contributions while not covered by BRS, the election remains effective until the member makes a new election.
4. (d)Agency automatic (1%) contributions for all members covered under this section and, if applicable, agency matching contributions attributable to employee contributions must begin at the time set forth in § 1600.19(c).