What the GA sees on the console isn't the same as what you see on the GIDS. Even though people disappear off of the GIDS, the complete list is visible to the GA, so it is easy for them to identify the next upgradable pax, even if he has boarded. On the other hand, there are many threads here stating that policy is "you board, you lose." In practice, it seems to be whatever the GA wants it to be. No one, to my knowledge, has ever officially quoted the policy from the company rule book, and the DL insiders seem to remain silent whenever this is discussed.