Originally Posted by
McFlyPHL
From what I've heard, online SDC actually works on DL. AA recently also banned SDC if your connecting city changes (e.g., if you're booked AAA-CLT-BBB, you can't SDC to AAA-PHL-BBB without paying change fees + fare diff). This was actually the last straw for me as I spent an extra 3-4 hours waiting around on my last two trips because of this.
You can change your middle airport on DL you just can't go from a connection to a direct. And in Y the fare class you purchased has to have availability on the new route at the time your request the SDC.