<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by fakecd:
Thanks Mwenenzi!
I usually "upgrade" the fare class when I make changes, and so I wasn't aware that I need to pay the difference with original fare.</font>
N class is the only one which you HAVE to upgrade to another fare type. All others don't HAVE to be upgraded, if there are seats in that class you can grab them and just pay the change fee.
Having said that though, if there is N class on the flight you want to change to then there will be O class as well, which is only about 2-3% more than O class. So the amount you have to pay won't be much (plus $33 change fee).
Dave