I would say if you are booked in the absolute lowest category, you have the best chance for upgrade.
People seem to spend a lot of time trying to predict these things on very little evidence. Yes, "full" is not supportive of an upgrade, but if you don't know the number of guarantee bookings, what category they are in, (we don't know) what category you are in, etc. There is a tremendous number of things crew sales departments do to get a ship to "full" status.