You have to get off the ship at the end of the first cruise. After all other passengers are off someone will take you off and you will get a new seapass. It only takes a few minutes. After that you can go ashore or stay on the ship. You should know what cabin you will be in when you book the back to back cruise. If you do book a different cabin on the second cruise, some one will move your luggage for you. I wouldn't change cabins unless they gave an upgrade.