I'm sure every airline's policies differ, but for Airtran customers, here is what just happened to me in the last 24 hours.
We got home from work yesterday and had a phone message from AirTran stating that our flight from PHL to TPA in April had had a "significant" change in time.
When I checked the website, the flight had moved back over 5 hours (a 2 PM arrival was now about 7:15PM) - that would put us in WDW after 9PM. Traveling with three little ones, that is NOT a good thing.
So I called AirTran to find out what our options were, and found that the only other non-stop left Philly at 6AM that day - also not good. The flights with stops all had significant layovers in Atlanta - none arriving before 5PM now.
Since there wasn't really anything else acceptable, AirTran offered me the option of cancelling reservation and getting a full refund.
A few minutes of searching found great times and fares for NonStop flights on USAirways from Philly to Orlando.
Our return flight is still Southwest out of Tampa, but I also found it cheaper to rent the cars from National in Orlando and drop them off in Tampa than a round-trip rental at either MCO or TPA.
Long story short (too late)
Because AirTran changed their flight, I was able to get better timing for the flight into Orlando - it will get us to the resort about 60 minutes earlier than we would have, and we saved $380.00 on airfare and about $60 on the car rental.
I feel VERY lucky, but just wanted to point out that there are times when a change by the airline can run in your favor.
Ted