I'm hoping the experts on this board will be able to answer this question for once and for all......

How do the various airlines (as they all seem to work differently) calculate tax on a re-issue do they;

1. Add up all the tax on as it stands on the day of the re-issue (taking into account all new fuel surcharges etc) and then minus the tax you had paid up until that point.

Or


2. Look at any new departing cities and/or flights and just charge for those.

Or

3. Something completely different.

As I mentioned in another thread AA will charge either no tax or ridiculously little for a re-issue (usually because of immense fatigue of all those involved) or the other extreme is Cathay who always seem very diligent but equally always come up with high numbers.

Whenever I ask the airlines their answers just keep a shroud of mystery on the whole process.