I recently had to change/cancel a flight and did so via the web site and I was pleasantly surprised as to how easy/efficient it was to do so. My original flight was PHX-BWI and I had to change it to PHX-MKE and extend the trip by 1 day. I was able to click on "change" on the reservation and it allowed me to search for new flights, showed me the new prices and what I would owe (in this case only the $150 change fee).

It went much better than my expectations (admittedly low). Actually it went much easier than my phone call to the Gold line that simply involved adding my middle name to my account. That call took a good 10-15 minutes with this lady who gave me the feeling that my account could have ended up in someone else's name. Fortunately when I looked today, all was well. Phew.