Originally Posted by
Grogs
You can definitely buy up afterwards, but there are catches. AFAIK, you're not actually buying an F ticket, just the right to sit in F on those particular flights. If you have to change flights due to weather or something, you could end up back in coach with little or no refund.
That's not true. You absolutely can call in and just buy up to an F ticket. The thing to avoid is clicking on the buy up offers online, those can throw things. Call reservations and pay for a buy up. It will be a real first class ticket.