No, you cannot simply sit in an empty first-class seat on a flight, even if the cabin remains unoccupied after the "Boarding Complete" announcement. Airlines have a grounded and strict "Self-Upgrade" policy designed to protect their revenue and the exclusivity of the premium cabin. If a flight attendant notices you in a seat for which you haven't paid or been officially upgraded, they will ask you to return to your ticketed cabin immediately. A "hard-fail" attempt to self-upgrade can even result in being flagged by the airline for a "code of conduct" violation. For a more supportive approach, if you notice a seat is empty, you can politely ask a flight attendant if a paid upgrade is available after the seatbelt sign is turned off. In 2026, some airlines may offer a "last-minute" discounted upgrade fee via their app mid-flight, but simply sitting down without permission is a breach of the "Gold Standard" of aviation etiquette and will almost certainly be discovered by the crew during their regular passenger manifest checks.