We always book "guarantee", and have been upgraded at least 75% of the time. More often than not, up a deck or two, or more, but occasionally we have made it into the next cat of cabin altogether. I've gotten the assignment as early as "several months", and as late as "checking in at the pier". Right now, I've got two guarantee reservations, but they are still way out there. I really don't expect cabin assignment until a month or less from sailing.