The best time to ask for a flight upgrade is immediately after booking, but opportunities extend up until boarding. Most upgrades happen for those who pay early or are frequent fliers with elite status. A paid economy-plus upgrade is more likely than a business or first-class upgrade on the plane. Upgrades to business or first class are extremely coveted, especially on long-haul flights, and so they're hard to come by. You're unlikely to get anywhere by asking for a free upgrade, but you can see whether a paid upgrade is possible. The best time to ask for a flight upgrade is right after booking, but if that doesn't work you'll still have a couple more opportunities to try for an upgrade. Business class airplane seat. jun xu / Getty Images Flight Tip Always explore the possibility of purchasing a flight upgrade immediately after booking and continue monitoring opportunities until boarding, as this increases your chances of securing a more comfortable seat.…