In "The Giver", at night after their ceremony the license plates on the bikes of the twelves were changed to ones that indicated what?
During the night after their Ceremony, the nameplate on the bicycle of each of the Twelves is "removed by the Maintenance Crew and replaced with the style that indicate(s) citizen-in-training" (Chapter 10). Although they will still go to school, the Twelves are no longer considered children, as indicated by the words spoken to them when they received their life assignments, "Thank you for your childhood" (Chapter 7). The Twelves have new identities, and these are represented by the nameplates on their bicycles. The Twelves are entering into their adult lives, and will be learning the carefully chosen and defined skills which they will each perform in the near future as productive citizens of the community.
This change also signifies their step into the "sameness" desired by the community. They are not individuals, they are part of the whole.