DEM, getting upgraded from your guaranteed category is never sure thing. Sometimes it has to do with the time of year you're sailing. Getting upgraded in September, October, or the two weeks before Christmas is more likely than if you were sailing during Spring Break time, or family vacation time. Sometimes upgrades have to do with how early you book, or even how much business your travel agency does with that cruise line.
You always have to be prepared to sail in the category in which you're booked, and hope for an upgrade.