The catch if you buy extra segments in a lower class of service is that the airline picks which segments you have bought and which are in your *ONE* fare. So you could wind up with L on the trans-continental segment, and use the D/A on that single-class JFK-BOS CRJ flight. AA seems the most reasonable about this, while BA usually (but not always) insists that the first 6 segments for NA are in the fare and the purchased ones come after that (no matter when they were actually purchased).