instead of selecting a specific room, you boot a gty ( which is usually priced lower than any cabin) which promises that you will receive a cabin in that class or higher. For example, on my last cruise, I booked a gty inside room cat n and paid $500 pp. I ended up with a cat FF outside room was was going for about $650 pp. so I paid $500 for a $650 cabin. now of couse there is always the chance that you end up with the lowest level cabin on the ship but then again, you also paid the lowest price.
You can't do worse than you are guaranteed is the basic premise here. I was lucky and was upgraded, but don't book lower that you will be happy with expecting an upgrade. My group all booked GTY. 2-booked gty 4a, and 6 booked gty-6a. The 4a got 6a rooms, and the 6a got 8b rooms. This paid off for the cruise line in the end, as 4 of us have decided we can never have less than a balcony again!! We booked our next cruise GTY 8a.
Remember, at popular time such as holidays and school vacations upgrades are few and far between. Also, it is very unusual to be upgraded beyond your category, usually inside to inside but a higher deck. Exceptions are booking the highest grade in a category, but then the lowest grade in the next category is usually only about $25-50 pp more.