There doesn't seem to be much rhyme or reason with how much additional money to collect, if any, when changing a ticket that is partially flown. Unfortunately, I've had to change return flights around about 5x this year, sometimes heading back sooner or later or changing the route all together. The first thing I do, if I'm within 24 hours, is see if SDC'ing is available. But even with that, I find the website offering more SDC options than the mobile app. When I do an outright change, the numbers presented online always differ than those on the phone. I did a change earlier this year; the website quoted a $1,300 difference which I was willing to pay, although the website error'ed out at the time it was capturing my payment details. When I called it in, the difference owed was only $260. Last night, I tried to SDC today for an earlier flight. The mobile app and website didn't offer the option. I called it in and the agent offered to make the change without the change fee for a $513 fare difference. I went to the website and was offered to make the fare change without the fee for $136 fare difference. And then sometimes, there's no add collect. I was ticketed EWR-RSW/RSW-MCO-EWR and changed the return to RSW-ORD-LAX/LAX-EWR. Shockingly, there was no additional collection. Which makes me think revenue management just makes up numbers to charge in these situations.