Originally Posted by
bharat91
Changes to a ticket should not allow a change to the fare class (I think?), and therefore I assume they are trying to rebook me into a Q fare.
No, that's entirely untrue. However, I suspect what's happening is that they're recomputing the fare based upon what you actually flew, since they didn't charge you the fare difference in the first place (despite being well within their rights to do so).
I agree with
WineCountryUA. You may be able to get the difference waived if you call, but probably not if you book it online.