It is not that easy. This part of the T&C usually doesn't apply because there are other rules overriding it. In general weero's description is right, however lately all fare rules contain explicit paragraphs about this. All fares I have checked don't reprice/recalculate w/o a route change.

It is also true, that a rerouting/reissue always lead to a recalculation of taxes. At least as long as I am flying. (Can be plus or minus) The only difference in the past was, that the amounts were usually that small, that the agents didn't bother to collect them. The whole problem really started w the famous YQ fuel surcharges as they can actually make a huge difference. Example: Switching from all UA code share flight numbers to all LH flight numbers on exactly the same flights can cost you several 100 USD going from the US to Africa.