United Airlines has made a change to basic economy fares, making them upgradeable after purchase. Passengers can pay a norefundable upgrade fee ‘switch to economy’ to make changes to their flights. Once you’ve paid this fee you can make changes to your ticket without any further fees (you pay the fare difference if the new flight is more expensive and receive travel credit if it’s cheaper). You can also cancel your ticket and receive a travel credit.
Hat tip to BalthazarOfNavarre