No, you cannot do that. You could cancel your existing booking and take the freebie instead, but you'd be subject to the cancellation provisions and whatever other fallout exists. Honestly I don't have the list of dates in front of me so the voyage you mentioned may not even be eligible.
It's not a situation where the cruise voucher has an assigned value and you can use it however you wish. There are very specific terms and conditions.
The free cruise is for up to 4 individuals in a CAT 8 room (ocean view.) You can pay to add more individuals and/or to upgrade the room class. But you cannot pay to extend the cruise (can't go from 3 night to 4 night, etc.) Also if you do not have 4 people sailing with you or you decide to take a shorter cruise, you do not get any credits or other compensation.
The cruise voucher does cover the cast member gratuities.