After reading this thread, it's not clear whether you should be charged the $100 change fee or not. I have tickets for Thanksgiving, EWR-STT, and I just checked and found they had gone from $560 to $428. I called CO to get the flight credit issued and was told I would be charged the change fee and would end up with a $32 credit.
Should I call back and try to find someone willing to issue the entire amount? Or just take what they are willing to offer?