From the formation menu, you can:
Swap CB, Swap S, Swap OLB
Swap DE, Swap DT, Swap ILB
You can assign your defensive players to matchup with any of the opponents offensive players, and there is also formation subs you can set.
If you haven't played this game, and are thinking about getting it, I
HIGHLY recommend it. Be warned though, you probably won't be able to play Madden ever again
