ashill

If the OP bought the cheapest fare available on a given flight and it wasn't fairly last minute (as is I think a reasonable default assumption), the odds are very good that the answer to the OP's question is simply "no". Southwest Airlines effectively allows this and Alaska Airlines did until a few months ago, but otherwise US airlines generally don't on most fares; the OP will have to pay the change fee (in cash, I believe, ie not as a value extracted from the fare difference) to get a credit for the fare difference.

If the OP bought a refundable fare, of course there is generally no change fee. And of course the fare rules give the answer with certainty.