Anne That's the other side of the "no change fee". Your DH got a discounted ticket in exchange for committing to a specific flight. Southwest lets a customer change, without any fees, if they're willing to pay the fare difference. SW will limit DINGS and other discounts to non-peak flights. They don't want the gate area filled with passengers that booked the cheap flight but want to standby for the flight time they really want.
If your DH's original flight is scheduled to be delayed, I think by at least 30 minutes, the GA may waive the fee.