I recently was successesful booking upgrades from LAX BKK LAX using United.com and a Star Alliance upgrade.

1. You have to have an issued ticket in Y or B, you can't upgrade a reservation that has not been ticketed.

2. In talking with the reservationist at TG, they were very clear that there are limited seats for *A upgrades on these flights. Even if the cabin is wide open, the upgrades are not limited. I asked if seats ever open and was told that typically they do not open additional seats for *A upgrades as the J cabin typically is very full. For example it is almost 100% booked between now and 10-Jan.