kkontny, I can't tell you much about the upgrade system; I've benn cruising with Princess for 30+ years and never had one. But then I always book very early (usually a year or more) and deposit a specific cabin. I have certain cabins and areas I like on the different ships. I don't like to leave it up to chance.
I find it interesting to hear Neil say the "system" is random. I would think they would want to favor their more loyal passengers; but then, maybe the more loyal passengers, like me, prefer to select their own cabins.