My TA tells me that the rules on this are quite clear, and apply to all published fares...

If you have already started to use the ticket then you do NOT have to pay any extra if the fare has changed. For MONE4 there is of course the USD75 (or equivalent) reroute charge, and the differences in taxes (in principle, though noone has ever tried to charge me the tax difference).

If you have NOT already started to use the ticket then the airline is entitled to charge you the fare difference.