Originally Posted by
Kmxu
I had no luck for SDC with Air China first segment (rest on UA). I ended with paying change fees ($120), but the agent got her supervisor to waive the fare difference (>$700 with U original fare) after she were unable to get my original routing back. I guess that Air China stopped selling ticket with UA segments (according to my search result). The new itinerary is all on UA.
Just a date point!
SDC does not work when one of the flights in the itinerary is being operated with an airline other than United or United Express.
SDC is a United perk and their partners are not obliged to waive fare differences but in your case, you were lucky.