When an airfare is lower and available, most airlines will credit the difference without the $75 fee in the form of a voucher if you do not change your itinerary. Its best to do this in person so you will not be charged the $75 fee.
When you decide to change your itinerary, (ie.flights), it is as if you are starting over. As a result of the change, you are charged the $75 change fee and then any increase in airfare (if any).