I booked the cheapest online ticket from SEA to ATL, which was
SEA/JFK/BWI/ATL. I assumed I could SDC the JFK/BWI/ATL portion to a direct flight (I didn't want to give up the upgrade SEA/JFK). However, I was told I couldn't do SDC because the segments were in different classes. Apparently, delta.com combined an SEA/BWI U fare with a BWI/ATL T fare. Looking at delta.com, it states "No changes to routing for fares that are flight specific or require specific routing." I'm not complaining--although I almost had a Comair-induced misconnection--since the fare was cheap. So a word to the wise: don't assume you can SDC a multicity itinerary into a direct flight.