corporate-wage-slave That all makes sense. But I’m not sure what you mean by “a fundamental fare difference”. Clearly if the fare has other components not included in the original ticket, like the different taxes you cite above, but what if the booking class changes? For example if the old flight was in O/Q class and the new flight is booked into V/L/M etc. Same cabin but higher fare, where taxes and all other components would stay the same but the base fare portion alone would change.

Actually it was AA codeshare of BA on a 001- ticket so more an AA question. (Sorry if that makes this an irrelevant question here: the mods moved this from TravelBuzz to the BA forum.)