Usually the airlines will waive the $75 change fee against a price reduction on the same flights as long as you take the reduction in the form of a voucher rather than cash. However, as soon as you ask for the flights to be changed, they usually require that you pay the $75. As for crediting the change fee against the lower fare, I've run into this a few times with the same kind of result you had here. The airlines seem to consider the $75 change fee as an entirely separate transaction from the ticket purchase. I had a Midwest Express ticket that originally cost me $300. When I changed it for a $285 flight, they took my $300 voucher cover the new flight, didn't return anything for the $15 difference (forfeited because of purchasing a lower fare ticket), then charged me $75 for the change.