Welcome! It is much cheaper to pre-book the packages as it is discounted and it includes the service charge. However you are limited to the "dining experience" menu, NOT the full menu. You can, of course, choose the restaurants a la carte onboard but you will end up paying more.
As far as the packages, no one has reported being able to switch around. To note, Hola tacos is owned and operated by an independent company so they don't sub out.