[Silent Bob]

Though he put it snidely, most airlines do charge an additional cost plus change fee, believe me I've paid my share. Mostly likely the same seat class of service wasn't available, only the higher class, hence you have to pay more. What you can do is call back and speak with another representative. Because it didn't work out with one, may work with another. Explain to them your situation, let them know you have documented proof and if its something of importance they may waive one fee but hardly ever both (most airlines will waive the change fee, but sometimes you will get lucky).

I've only done this once before with United, but I was charged in a similar fashion and i said to the agent "man, with that amount I can upgrade and with miles". To which I called UA MP, explained to them my situation and used my miles for the business first seat and a change of date.

Good luck, hope it all works out in the end.