If you've booked a cabin that you really
want just ask your TA or your Princess PVP to put "no upgrade" on your booking. Even if you've booked a specific cabin there is
a chance that you'll be "upgraded" unless you specify "no upgrade".
Sometimes the "upgrade" is not so great
. You might go from an amidships cabin to a cabin a deck above that's way forward or aft, or under the pool deck or galley (which we, personally, wouldn't want). While it does
happen, it's very unusual to be upgraded out of the category you've booked (outside to balcony, for instance).
These days it's very
rare that a ship doesn't sail completely full. But, as Jimbo said, it can't hurt to ask when you check in!
Every cruise we've been on there have been signs everywhere
that there were no upgrades available for that sailing.