My experience is, change the flight in advance of your travel (i.e. before you arrive at the airport) and you're going to get saddled with the $75.
However, I had a nice surprise recently. I was booked on an American flight and had to change to a later departure the same day. Turned out the fare was $180 less for the first segment. They did charge me the $75, but also gave me the $105 difference in the form of a voucher.
But jms8090, if an airline is treating the fare difference as nonrefundable, why doin't you simply have them refund the entire ticket and apply the voucher (minus $75) toward the new booking. Preferably this procedure should be done in front of the person who won't give you difference to begin with.