Welcome back, Joann!
I hope your vacation to the most magical place on earth has started off swell! You are going to love the visually stunning Rivers of Light. You've also made an excellent choice in taking advantage of a
Dining Package. I've had the opportunity to experience a Rivers of Light Dining Package for both Tiffins and Tusker House Restaurant, Tiffins is now one of my new favorites for unique dining in Walt Disney World® Resort.
Rivers of Light Dining Packages run on their own unique reservation system, as there are only a certain number of reservations allocated each day for guests wanting a dining package to ensure reserved seating for Rivers of Light. A standard Advanced Dining Reservation at the same restaurant for a more desirable time will not offer you the ability to take advantage of a "Dining Package," or the reserved seating voucher that is attached to your reservation.
However, guests' plans change frequently, with many of them modifying or cancelling within the 24 hour period to avoid the cancellation fee, making it possible for you to modify your existing reservation to a more desirable time.
There are a few ways you can check for availability: on the My Disney Experience App from your smartphone, stopping by the Concierge Desk of your resort hotel, or even stopping by Guests Services at the entrance of Animal Kingdom to check day-of availability.
I hope you are able to adjust your reservation, Joann! I also hope you have a wild time!
Faith