If I receive cards in a Prerelease prior to the expansion set's release date, can I use cards from that set in a sanctioned tournament as long as those cards are reprints of already legal cards?
Per the current Play! Pokemon Tournament Rules Handbook you cannot use any new cards until the set is considered legal for tournament play (currently the 2nd Friday following that expansion's US market release date), however you can use a newer version of a reprinted card that is currently playable in the Standard or Expanded format immediately upon its official release.
Source: TPCi Rules Team (2022-08-04); TPCi Rules Team (2024-01-25)