In air travel, nothing is a done deal until it’s been done.
Take, for example, your seat assignment. You may believe that, since you booked seat 7A, it is confirmed and guaranteed to be yours. However, as Ann Coulter was surprised to learn last year, airlines can change your seat at any time — even if you thought you paid specifically for one.
When you buy an airline ticket, you sign a contract of carriage. This contract allows the airline to change many aspects of the flight without telling you. For example, Delta’s contract of carriage states that the airline “may substitute alternate carriers or aircraft, delay or cancel flights, change seat assignments, and alter or omit stopping places shown on the ticket at any time.” So while you have purchased transportation, don't feel so certain that you purchased that particular seat.
When airlines change the aircraft on which they are operating a flight, the layout of the cabin can also change — which can affect seating arrangements. There may be fewer premium economy seats available or one row may have less legroom than expected. Airline staff may also change seating arrangements to accommodate families traveling together.
A Delta spokesperson told Travel + Leisure that the airline “makes every effort to ensure customers fly in the seat and product they paid for no matter how long in advance (up to 331 days) or through which channel they purchased their fare.” Passengers who discover that their seat has been changed should contact customer service for assistance.
If you have researched your seat, paid money for it, and would like very much to be in that specific location, keep records of your purchases. Continue to routinely check your flight information in the lead-up to takeoff. If anything changes, contact customer service. They will likely offer you compensation for your troubles — especially if you approach with proof and remain polite.