Originally Posted by Ruhr

I've flown itineraries like these with one segment on a single cabin RJ in Y, and the SDC options put me in the front cabin, not the back (because, I've bought an A class ticket). The tricky part is getting UA to change only BBB-CCC before you even start your trip. Check your app to see what kind of routings you get from AAA-CCC within 24 hours of your AAA-BBB flight. Otherwise, you might need to call just to change the BBB-CCC part. It may be possible with a skilled agent.

The other thing you can do is check in, and fly AAA-BBB. While in the air on your way to BBB, if your flight has WiFi, start checking your app for SDC changes for the BBB-CCC part. You might find an SDC that gets you out of your RJ flight. Uh, if you do this, please don't have any checked bags, right? Right then!

Good luck!