Suppose I have a unrestricted ticket. It allows free changes and cancellations and says "fares must be recalculated using historical fares in effect on purchase date" (or words to that effect) in the rules.

If I have a J class A-B-C-B-A ticket and have flown A-B-C already and want to change the return date for C-B-A, how would the new fare be calculated?

I always thought that there was no fare difference if the fare class stays the same but are there times this isn't true?

Do the airlines have a computer that can actually calculate what I would have paid for A-B-C-B-A on my new C-B-A travel dates if I had booked it way back when I did?

I tried to change the ticket from Expedia, where I booked it. Their idea was to issue me a 'new' C-B-A ticket (for 5x what I paid for my original A-B-C-B-A ticket) and to give me the residual value for my original ticket (which was zip, the RT was cheaper than the OW).

Then I called the airline, AA, who kept giving me a $3,000 fare difference. If I had booked the ticket on my new date, I know
the fare would have been exactly the same; when I was originally booking, I checked prices with both return dates, because I knew I might have to change it to the new date. They were the same.

The fare class is the same, the flights are the same, so what gives? I am assuming there is something I am missing?

Thanks!