It is possible, though unlikely, for a cabin to be changed after it is assigned.
Once you arrive at port, if there are still cabins available, you can request an upgrade at the Upgrade desk. The cost of the upgrade will be calculated based on the cost of the category you initially booked -- for Guarantee, I think the upgrade cost is calculated as if you booked a 9D (the least expensive Oceanview room on the Magic).
When you complete your Online Check-In, you will select a Port Arrival Time (PAT), which will also give you a Boarding Group number. The PAT is the earliest you should arrive at the terminal. When you arrive at terminal, you can give checked luggage to a porter (who will expect a tip) -- this luggage will be delivered to your room, generally by 5 pm but possibly later, so make sure to keep any essential medicines, dinner clothes, and anything else you'll need before dinner with you. Your boarding ticket (generated during Online Check-in) may be checked before you're allowed in. You will go through a security scanner. You will then check in by showing the boarding ticket for the group and identity documents (birth certificate/photo ID or passport) for each guest. You can then sit down and wait to board. While you're in the terminal, make sure you've downloaded the
DCL app. The app should switch from "planning" to "onboard mode" so even guests without a Disney account can log in using their room number, name, and birthdate. The app will tell you everything happening onboard that day and on future days.
When boarding begins, Boarding Groups will be called to line up and board in order. The groups are typically called 10-15 minutes apart. Once onboard, you can get lunch at Cabanas on the pool deck or at the sit-down restaurant that's open, go swimming, tour the spa area, or just sit down and wait for rooms to open around 2 pm.
My primary tip for new cruisers is to recognize that you won't be able to do everything, and that you should just enjoy what you do. If there's anything you tried to book in advance but couldn't, you can inquire at Guest Services (for excursions and activities) or the designated Dining Changes location listed in the DCL app (for adult dining). Then just relax and see what you feel like doing each day during your cruise.
