Originally Posted by Snowdevil
Yes, there is.
While -U- inventory is capacity controlled (as well as -A- inventory for First Class Saver Awards), those capacity controls are lifted within 24 hours of that flight's departure time.
So if there are six -F- inventory seats available for a flight with 24 hours from now, you won't see a lesser number available in the subsets like -U- or -A- inventory. You would be able to get those six seats in -U- class (upgrades) or -A- class (First Class Saver Awards).
Wow - old post! Let me amend it slightly since things have changed a bit.
Typically the capacity controls on the First Class cabin are lifted within 48 hours of a flight's departure time. There are exceptions to this though - primarily on flights to/from the East Coast, ORD, and I believe CUN as well.
On those flights - as unpopular as this will be to say - AS realizes that there's a much greater likelihood of selling an -F- seat instead of allowing it to be booked as a comp upgrade. Consequently, the time before departure where capacity controls on the F/C cabin are lifted varies. Sometimes it's 24 hours. Sometimes they don't lift it until the day of the flight. It just depends on the market and how the flight has sold in the past.