Originally Posted by
trueblu
-- was wondering whether I can 'buy-up' first for a (relatively) small fee on the day, as has been reported here --
If you are thinking about the LFBU upgrades that are sometimes available on the day of travel, these will not work for you.
They only give you miles/points based on the original fare bucket you were booked in and in any case, they will not be offered to you as you are already seated in F.