If you forget to set your lineup for a Friends Mode Championship prior to one of these game start times, you'll still be able to edit your lineup, but would have to choose from players that have not started their game yet.
Essentially, this works the same way as a typical Late Swap contests would. Please keep in mind, consistent with our Late Swap rules, once we've reached a games scheduled start time, you would no longer be able to select players from that game.
You can read more about the Late Swap feature here.