Quick Swap first, always. Then Full Swap when news has changed how the slate should be played.
Quick Swap removes out players and replaces them while keeping the rest of your lineup intact. It takes seconds and it's the floor of post-lock work, because a player scoring zero does more damage than almost anything else. Do it every time the icon turns red.
Full Swap rebuilds every open slot in your lineups from new simulations. Use it when the slate has shifted, not just when a player is out: a starter ruled out changes who gets touches, how ownership reacts, and what stacks make sense. NBA is where this matters most, since one injury can flip the slate. In sports with less late news, like MLB and NFL, Quick Swap alone often does the job.
As a very general rule of thumb, follow the various lineup-alerts channels in Discord. If a “significant projection change” sim has run, it’s a good indicator that you will be best served by running a Full Swap.
Related: How do I Quick Swap? · How do I Full Swap? · Saber School: Scale Your Swaps