Suzy, I just booked, online, the excursions we chose for our January cruise on the Jewel. Unlike Princess where the excursions were not charged to our card until after the cruise started, RCCL charges them to your credit card immediately upon booking.
I called RCCL yesterday and was told that if we wanted to cancel an excursion I could just call them and it would be credited back to my account. It appears from the website that the cancellation could also be done online, but the representative didn't mention that.
I like to book the excursions prior to sailing, but several times have changed my mind once we got on the ship and talked to people at the excursion desk. They have always been very helpful and there was no problem with cancelling one excursion and booking another. This being said, it's important to remember that they do have a time frame during which the excursion cannot be cancelled without penalty. This is usually within 24 hours of the excursion, but I suppose that could vary based on the excursion.
Enjoy choosing your excursions! It's part of the pre-planning that builds excitement for your cruise.