Generally, you are expected to sit in the seat assigned on your boarding pass, but switching is allowed under certain conditions. For safety and weight distribution reasons, you should never switch seats before takeoff without asking a flight attendant, especially on smaller planes. Once the "Fasten Seatbelt" sign is turned off, you can usually move to an empty seat of the same class (e.g., Economy to Economy). However, moving to a "preferred" seat or a higher cabin (like Business Class) is not allowed without paying. If you want to swap with another passenger to sit with a companion, the polite etiquette is to offer a "like-for-like" or better seat (e.g., offering an aisle for an aisle). Always accept a "no" gracefully, as some people pay extra for specific seats.