On our first Alaska cruise on Celebrity Mercury we were upgraded from a Cat 1 to a Sky Suite. That was a nice upgrade.
On the Ocean Princess we were upgraded from a Cat BA to a Cat AB suite. That was also a nice upgrade.
On the Norwegian Star we were upgraded from a BA to an AF mini-suite.
I really think it is the luck of the draw. We haven't been upgraded in a number of cruises and I don't mind. I always book in the category I want and if we go up that's great.
I also don't consider it a true upgrade unless it is from balcony to suite (mini suite). We only book balconies but I would consider inside to outside and outside to balcony a true upgrade.