Seat assignments, no matter the fare class you purchased, aren't guaranteed and could change without notice because of schedule changes, equipment swaps or other unforeseen circumstances.

Yes, United Airlines reserves the right to change your seat assignment at any time for operational, safety, or security reasons, as stated in their Contract of Carriage. While it is frustrating, common triggers for seat changes include aircraft swaps (where the new plane has a different seating configuration), weight and balance requirements, or the need to accommodate passengers with disabilities or families with small children. If you paid for a "Preferred" or "Economy Plus" seat and are moved to a standard seat, you are entitled to a refund of the seat fee. United's system usually attempts to reassign you to a similar seat (e.g., swapping one aisle for another), but it is a "pro-tip" to check your reservation frequently via the mobile app, especially in the 72 hours leading up to departure. If you find your seat has been moved, you can often go back into the seat map and select a new one for free if a better option has opened up.