When changing AA flights, how is "fare difference" calculated?
 

I originally booked PEK-DFW-SFO (AA 262-2397) on 10/1 on fare code M (because that's the cheapest I can get 100% credit on AS). Now because of some passport issue I have to change that to 10/10. I changed to exact same flight and exact same fare class, but they still somehow calculated $600+ fare difference (and there doesn't include additional ~$250 change fee).

How does $600+ fare difference calculated? I thought for exact same flight and fare class, the fare difference should be minimal? Is that normal?

Assuming this is a one way ticket or is the outbound of a round trip ticket, then the fare difference is looking at how much the ticket costs to buy a ticket now for that journey minus the amount already paid

No this is the inbound of a roundtrip/multi-city (I already flew SFO-DFW-PVG).

(Looking at ita, PEK-DFW-SFO on M on 10/10 costs $900+ right now. My original ticket was ~$1,400)

If it's the inbound, and in the same booking class, then effectively there should be no fare difference. Technically the fare on that segment is typically calculated based on the fares in effect at the time you purchased the fare, but there would need to be that particular language in the fare rules (which is typical). You did save the fare rules, right? ;)

One wrinkle - to/from Asia, fares are almost always different for Mon-Thurs vs Fri-Sun (or similar). But probably not $600 different in coach.

Did the fare you purchase have any requirements that are no longer being met - e.g. minimum stay requirements

I have no idea. How could I check?

Also my original ticket has ~2week stay and my new ticket has ~3week stay.

I think that you need to phone and ask what the fare basis is and then ask for details of fare basis for the new fare

It isn't possible to get close to a definitive answer without the details of what was paid and its rules

Thanks. I actually just used the web form to email the customer relation team for an explanation of the fare difference.

Good luck with that one! For one they, C.R. does not generally involve itself in issues before the travel has completed. For another, well, read the many posts...

Agreed, only Reservations will help. Remind them that they should be calculating the fare difference based on fares that were in effect at the time of the original ticket purchase.

I doubt that moving from a 2- to 3- week trip meant breaking a max stay requirement.

Only difference I can think of would be a day of week restriction, but that should be small.

OP - what date did you purchase your ticket? Someone here can look up the different M fares available on that day.

The first email was on 6/28, but I vaguely remembered I was having some trouble to get the confirmation email after I made the booking, so the actual booking date might be a few days earlier than 6/28 (but not more than 1 week)

Phone and ask what the fare basis is
If you post the exact details of trip, may be possible to work out whether AA is likely quoting the correct fare difference