I'm not sure I follow/agree with millionmiler. Before you fly your first segment, any changes you make amount to turning in your ticket and buying a new one. Nothing is locked in. If your old ticket was issued in a cheap place and you're standing in an expensive place, you will pay the expensive price.

OTOH if you've flown at least one segment (international? intercontinental?) *everything* is locked in at the prices in effect at the place/time where/when you bought the original ticket. Add a continent, change class of service, etc. all at the original price structure, even if prices have gone up. The only exception seems to be taxes (out of the airlines' hands) and certain fees like fuel surcharges (because they seem to be getting away with it.)

Did I get that right?