Sometimes I've had this work - other times for your original flight and cabin, it will show "Original Flight" in the cabin block and won't let you select it to rebook the same flight and cabin at the lower fare. Experienced this on both DL and AA. The ways around this are:
1) Call and have an agent do it (takes time)
2) Switch to a different flight then immediately switch again (risk that fare changes on your original flight before you can rebook back to it or that the booking system was returning a phantom fare)
3) Completely cancel, take the ecredit, and rebook a new reservation (same risk as above about the fare potentially going away)
4) With AA, book the same itinerary as a completely new reservation but then use the 24 hour "Hold" and once that goes through, cancel the original reservation and then use the Trip Credit to then go back and pay for the reservation on hold.