Originally Posted by
frb98mf
Bear in mind UA and the other US carriers have a typical policy of filling their forward cabins by rolling up their frequent fliers either as a comp or using various upgrade vouchers, and often those on flexible tickets. A large number of them can get this cleared >24h before travel, so if you're looking on day of travel, that might be why the biz cabin is so full.
Not on international routes. On domestic routes (though not even always transcons) and short international routes.
On UA, the way to sit up front is to pay for the ticket (money or miles) or to PAY to upgrade (money, certificate, miles), and the upgrade (at least EWR-TLV) is notoriously difficult to clear.
The only way they're bumping someone up (OpUp) is if Y is oversold but there's an empty seat in C. Very rare and not really an official policy of any kind.
Same with DL and AA (at lease conceptually)