Originally Posted by Fiumicino
Ok, I have upgraded my fare once in 2005 with CO and they didn't even suggest I should pay a change fee; buying up is free with UA (it is written in the tariff; I cannot say if all tariff has it but in my experience it seems so). AC is also free.
Depends which fare you purchase but here are AC's rules for their lower fares:
# Changes are permitted as follows:
Prior to day of departure - CA$30 /GBP 14 / US$22 plus taxes and any fare difference if applicable.