Upgrade to what? You're in a veranda room already.
Cat 5 is the same room, different deck (Cat 5 is on deck 7). Your cat 6 is on either deck 5 or 6. In reality - going from a 6 to a 5 isn't an upgrade, it's a room change.
Cat 4 gives you an extra 30 sq feet and you can sleep a max of 5 instead of 4. Pretty much a room change again, this time up to deck 8. OK so this is more than just a room change, you move up to the high rent district (location, location, location) where the suites are.
Then there are the real big upgrade to cat 3 or better - these are all suites. And there are perks that come with living here. The problem with getting upgraded to here is that usually there is a waiting list for all of these rooms (go figure, the cheepest-cat 10,11 & 12 and the most expensive-cat 1, 2, 3 are the first rooms to go and are the most popular).
Bottom line - is it impossible for you to get upgraded. NO, it's not impossible, but its not likely for ANYONE to get upgraded. You'll have a slightly better chance of getting upgraded if you are sailing between now and spring break - on a seven night cruise.