Confused about airlines’ policies regarding schedule changes? Here are the rules for the three largest U.S. carriers.

AMERICAN If the change exceeds 90 minutes, you can switch (with no fee) 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. American allows full refunds if the change is more than an hour or if a nonstop flight becomes a connecting itinerary.

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DELTA The airline allows free changes if the new itinerary 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 nonstop trip now has a connection.

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UNITED If the 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 arrival-time changes exceed two hours, you can cancel and get a refund. If the departure time shifts, no change is allowed if the flier still arrives at the same time.