Changing carriers -- fee ?
 

I know changing carriers is supposed to be free, but is there ever any YQ extra charge? Suppose you buy a flight on one carrier (like AA) and then later decide you'd like to fly the same route on BA. Perhaps if you originally bought the flight with BA, the YQ might have been higher. If you change it to BA later, do they ever recalculate the YQ and ask you to pay more?

While you should not be charged the change fee, you are responsible for any difference in fees, taxes, and carrier surcharges caused by your change of carrier. So if switching to BA you should be prepared to pay the BA surcharges.

See, for example:
https://www.flyertalk.com/forum/onew...de-change.html

Thank you.

Does the free codeshare change apply only to xONEx tickets, or does it apply to any tickets with codesharing available? I've seen some pretty crazy price differences between codeshares on the same flight before, and it'd be nice to be able to purchase the cheapest one and then just change to the preferred codeshare for my FFP.

The rule that is being referenced in this thread is an xonex rule and is not necessarily related to codeshares nor does it have any relation to normal point to point or round trip tickets. Changes you would be trying to make to point to point or rt ticket would be governed by the fares rules of that ticket.