Originally Posted by Bukhara
Tax levels can vary from airport to airport - particularly LGW and LHR - so if someone is doing their job by the book, they should charge you the appropriate levels (and refund where necessary).
In practice, this rarely happens unless you make a change to your booking that requires a reissue, such as a re-route.
I believe you aren't entitled to a refund, although I don't know the rules well enough to be sure of that. I think I do remember that if you've bought an extra segment and later delete it, you do not get the money back - that's different from taxes, of course. AA has add-collected taxes at a couple of stations, and the MCO of course says "you ain't ever gonna see this money again" or words to that effect.
And on the OP topic of co-terminals, for those who don't read and memorize everything written here, AA does/did not consider the airports in the LA basin as co-terminals. I tried to move a LAX departure to ONT and they wouldn't do it without a reissue. "Yep, they're co-terminals for ordinary tickets, but not for ONE," they said. That was about two years ago, and the ruling was delivered by one of the senior folks on their rtw desk.