I have a round trip reservation with transfers on each segment. I looked it up on the website, clicked 'Change Flights', and then it took me to a page with 'Flight Details' and two checkboxes—one for each leg. So I checked only the second one and then clicked 'Continue'. It offered a search box, and I searched, and then selected a new itinerary for this segment. Then it came back with a summary of the whole itinerary.

The new whole itinerary includes the selection I just made, but the first segment—the one I did not select with the checkbox—has its fare classes changed (same flights still).

Is there any way to avoid this? This seems like a bug to me. I'd guess somehow the fare rules are causing it, but I don't understand why it would display a result that breaks the original fare rules.