Yes - sometimes. But you shouldn't count on it happening. The common wisdom around here is to book the flight you need at the best price you can get now, and then keep watching it and if it goes down apply the full value of your original ticket to the new one and pocket the difference as a 1yr travel credit that can be used for future flights (assuming the original fare was not refundable). This is a big benefit of Southwest's lack of change fees (although some other carriers let you reprice and get a voucher, but not with the ease of Southwest or with the ability to change to a different routing or day).