Holding back seats for revenue is quite common on US. Keep in mind mileage upgrades and status upgrades come out of the same fare bucket. US East seems to hold about 25% of the seats for revenue until the day of departure. (This is based on status upgrades at the 7 day window....more seats may be held further out) I have seen US West hold 10/12 seats for revenue up until 5 days before departure. Seats are released for upgrade periodicly as you get closer to departure. I would keep calling back to check for availability. Last week I called to upgrade a friend who has no status for a flight the next day and they were holding 7 seats for revenue. I asked again and told them the upgrade was important as my friend could not use status to upgrade. The CP desk made a call to inventory management and they opened up a seat for her. ^ So definitely call back and keep checking for availability.