Agree, swapping tickets doesn't make sense. And you likely can't after using the outbound segment.

But if you cancel the return segment in advance, you retain the value to be used in the future, subject to a change fee. For a return, it may only be usable in the future between the same city pairs (rules depend on the airline), but it doesn't hurt to have a credit on file, and it's better to have then not. It expires after a year.