I have bought a B fare and upgraded to C using my UA miles on LAX-BKK-LAX in the past. I always checked KVS before I buy the ticket, just to make sure that I could lock in the upgrade. I definitely noticed the amount of seats available got tighter. I was looking for a friend a few weeks ago to travel LAX BKK during Thanksgiving and there was only one seat available during a 10-day period.

So, just like what KVS says, the seats are there, just not that many of them. Good luck!