No. They also don't ask you to pay the difference if prices increase between the time you buy and the day you fly.

Some airlines will permit you to rebook at the lower rate, if you pay a change fee. The change fee (often $100+) will usually exceed any price drops.

Southwest does offer full credit (no change fee) on future travel if you cancel a ticket, but I'm not familiar with the details on this. If there is a delay receiving the credit, you won't be able to use it toward the immediately rebooked ticket.