If you have etickets, just call the reservation center and tell them you think your fare has gone down. If the agent is experienced, he/she will know what to do and issue you e-vouchers for the fare difference and reissue your tickets in the new class and fare.

If you have paper tickets, still call reservations and they will set everything up for you. Then go to the airport or CTO (I prefer CTO), and they will issue e-vouchers and reissue new paper tickets for you. Be sure to bring your original tickets with you.

If someone insists that there is a change fee, say thanks and hang up, then call back and ask to do the downgrade (airline lingo for what you want), and you will most likely succeed.

Golden Rule of Airlines: If they say no, call back and ask again.