No, you cannot simply "self-upgrade" to an empty first-class seat, even if the flight has taken off and the seat remains vacant. Airlines treat seats as a tiered product, and sitting in a seat you didn't pay for is often viewed by crews as a form of "theft of service." Flight attendants have a digital manifest on their tablets that shows exactly which seats should be occupied; if you move without permission, you will be politely (or firmly) asked to return to your assigned seat. In 2026, most U.S. carriers have strict rules against on-board upgrades to maintain the value of their premium cabins and to reward loyal passengers who are often on a "waitlist" for those empty spots. If you truly want a better seat, the best "high-value" strategy is to ask a flight attendant after the doors have closed. Occasionally, if there is a weight-and-balance issue or if your original seat is broken, they may move you as a courtesy. However, doing so without asking is a quick way to cause an awkward confrontation or, in extreme cases, be met by security upon landing for failing to follow crew instructions.