What to do if your seat is moved by the Airline
(Relevant to the U.S. Airline Passenger Carriage)
Sometimes, when you travel with U.S. airlines, you will be surprised to know that the seat you had booked was moved and was given to another person. Not Fair? Yes!
But, admittedly, the airline has to have a strong reason to take your booked seat. And this practice is not regular. As a matter of fact, when you buy your air ticket -- which in legalese is called "the contract of carriage" -- you agree with the airline's conditions and terms. The airlines -- for example, Delta, United, American, and others -- include legal provisions allowing them to substitute alternate carriers or aircraft, to delay or cancel flights, to change seat assignments, and to alter or omit stopping places shown on the ticket at any time, etc.
If it happened to you, our advice is to stay calm and reasonable. As a matter of fact, aggression and emotions to an almost 100% certainty will not help in your case.
You should instead consider:
1. Calling the airline's customer care or contacting their representatives, and
2. Describing what has happened to you, and
3. Asking for a refund.