@Ms.Minnie
My flights were booked with Rapid Rewards points, so the fares are fully refundable. I cancel and rebook when I find a better price. Changing the flight is also possible but then the fees paid are no longer refundable. They would only be returned via a credit for a future flight (within 12 mo.) I typically have enough points to just book the new fare as soon as I see it and then go back and cancel my original flights. Otherwise, some risk of losing the flight exists.
I have not booked a flight on cash in a very long time.....but I think:
For cash flights, some are refundable some are not. Wanna Get Away (lowest priced) fares are not. However, you can change/cancel them and get a credit voucher, then use your credit to book the lower priced flight. Again, 12 month rule applies for the voucher balance (amount the flight went down) and if you don't use it you lose it.
BIG BUT: If you've booked Early Bird check in, you will lose it. I don't book Early Bird until I'm very satisfied with my flight price, if at all.