Whether or not you can switch to a different cruise and whether or not you have to pay a penalty for doing so is dependent on two things:
- Whether or not you booked a restricted rate. GT bookings, and certain other discounted bookings, can't be changed to a different date.
- The date of the booked cruise. It has to be far enough away that cruisers are not yet required to be paid in full. Your cruise is far enough out that this isn't an issue.
It doesn't matter if you've paid the whole cruise price in full, as long as you're before the required "paid in full" date, which is what most people are referring to when they say PIF.