ACtually the guarantees only become available in two instances.

As soon as the H cabins sell out they offer a number of guarantees. It may be only 2 or 3 cabins. These are then sold and are now almost considered a "waitlist" for H cabins. They know from experience a percentage will cancel. When someone cancels they move these guarantees into those cabins. If no one cancels then that person may get an upgrade. YOu cannot buy a guarantee if a cabin is available. So it is all a matter of timing. This is now happening in every category.

Sometimes they will offer a lot of guarantees if they want to sell a ship that is not full. It is a way to sell a lower end price. Happens on the less popular itineraries.

As to upgrading on board it all depends. Sometimes you do get a better rate depends on the ship, how full it is, how the Hotel Director handles the situation. Each ship can handle this their way.

These days I am more of a proponent of the cabin assignment. I used to really like the guarantees but now with them assigning the guarantees early on when lower level cabins open up I am not as fond of them. YOu still usually get what you pay for but with no choice. It is like rolling the dice in Vegas except now the odds are not as good as they used to be.