Airline schedule changes are often frustrating. Suddenly a mid-morning flight shifts to one leaving before sunrise or a leisurely layover turns into a mad dash to the next gate.

Savvy travelers, however, realize that sometimes schedule changes -- usually an hour or more -- allow the freedom to make itinerary adjustments without paying hundreds of dollars in fees.

Airlines typically publish schedules 11 months in advance. Southwest Airlines generally keeps to its schedule but American Airlines, Delta Air Lines and United Airlines make many changes during that time. Some shift flights by a minute or two. But others dramatically alter itineraries, and that's where fliers can benefit.

Let's say a family is flying from Orlando home to Boston. They really want the 4:30 p.m. flight to get a few extra hours at the pool. But that flight is US$60 more per person than the 9:00 a.m. flight. So they book the earlier, cheaper flight.

A few months pass and the schedule changes. That 9:00 a.m. flight now leaves at 7:15 a.m. Most passengers might grumble and accept the very earlier wakeup. But they shouldn't.

That big of a schedule change provides fliers the ability to change -- for free -- to almost any other flight the airline offers, including the costlier 4:30 p.m. flight originally desired. Sometimes passengers can even shift their flight a day earlier or later, as long as they depart within 24 hours of their original time. Normally, such changes would cost US$200 per passenger plus any difference of fare.

"The fact that you can change is generally made clear. They don't make it clear what you can change to," says Brett Snyder, who runs an air travel assistance company called Cranky Concierge. "The rules are crazy and complex."

A good rule of thumb is to call the airline if your flight shifts an hour or more. Travelers should also be aware of smaller changes that might turn a tight connection into one that breaks the airline's minimum amount of time allowed for a layover.

Minimum connection times vary by airline and airport and even by terminal. For instance, on domestic itineraries Delta allows as little as 35 minutes to connect in Atlanta, United 30 minutes in San Francisco and American 25 minutes in Phoenix. The general rule: if your connection has shrunk to under 45 minutes, call the airline.

"They tend to be pretty flexible as long as it is a legitimate change," Snyder says.

Fliers should be aware of alternatives. Go to the airline's website and search flights by schedule. When calling to for a change, ask for specific flights. Also, check other airlines. You might be able to get a refund and buy a new ticket on another carrier. However, airfare typically increases closer to the date of travel.

The following are the rules for the three largest U.S. carriers:

- American Airlines: If the change exceeds 90 minutes, the airline allows passengers to switch to any flight within 24 hours before or after the original departure time. If a nonstop flight becomes a connecting one, passengers can switch to another nonstop as close as possible to the original departure time. American also allows full refunds on tickets if the change is more than an hour or if a non-stop flight becomes a connecting itinerary.

- Delta Air Lines: Delta's policy is the most generous. It allows free changes if the trip departs 60 minutes earlier than originally scheduled or arrives 30 minutes later. Delta allows full refunds if the new itinerary doesn't arrive within 90 minutes of the originally scheduled time or if a non-stop trip now has a connection.

- United Airlines: The airline's rules only apply to the arrival time. If that arrival time shifts by at least an hour or if a nonstop flight becomes a connecting trip, passengers can make free changes to most other United flights. If that arrival time changes more than two hours, passengers also have the option to cancel and get a refund. If the departure time shifts, however, the airline's policy is that no change is allowed if the flier still arrives at the same time. Still, some fliers have had success getting free changes if the departure change is significant. It never hurts to ask. The rules are more restrictive for international trips that involve United's partners.