Ports VERY rarely have changed because of a hurricane. I recall a couple years ago where a swap was made between Tampa and Miami (I think, maybe it was the other way around). Carnival shuttled the people from the original port to the revised one. Air is a different matter. They aren't necessarily going to buy new air for you to get to the new port if, for instance, JAX was closed. I'd say they may offer reimbursement to you for the revised air you had to arrange.
In reality, the usual thing is the ship will not move to a backup debarkation port. It most likely would become a modified cruise of a shorter duration with a partial refund for those that went, or a total refund for those who cancelled or couldn't get to the ship.