Don't be disappointed if you don't get a free upgrade. They're very rare.
And nobody on this board has figured out a pattern for who gets upgrades and how to increase ones chances by booking early, paying early, or any other method. The only thing that seems to increase the chance of an upgrade is to be booked in GTY status for a category rather than into a specific stateroom. But GTY status can also lead to the worst room in a category rather than to an upgrade. And if someone waits until there are only GTY rooms available for a category, chances are they've waited too long to book and they're paying much more than if they had booked early.
Always expect the room or category that you're booked in. Then, in the very unlikely case that you get an upgrade, be happy. That's much better than expecting an upgrade and the being disappointed when you get exactly what you booked after all.