No, in 2026, you generally cannot move to a First Class seat just because it appears empty after takeoff. Airlines view the premium cabin as a distinct product with a significantly higher price point, and "self-upgrading" is considered a violation of the contract of carriage. If you sit in a premium seat without authorization, flight attendants will politely but firmly ask you to return to your assigned seat. In some cases, attempting to "poach" a seat can lead to being flagged in the airline's passenger system. If you see an empty seat, the proper procedure is to ask a flight attendant if a paid or mileage-based upgrade is available mid-flight, though this is rare on international routes due to catering and staffing logistics. For domestic flights, most of those "empty" seats are actually reserved for elite frequent flyers on a waitlist who are cleared automatically by the system. The only time a move might be permitted is if your original seat is broken or if there is a critical safety issue, and even then, the crew will usually move you to a seat in the same class of service.