You do not have to make new dining reservations. It's really not necessary to have them linked or designate that you are on a dining plan at the reservation stage, anyway (with the exception of prepaid reservations - CRT or dinner shows. If you have already paid for those, you can switch your payment to dining credits either when you arrive at CRT or when you pick up tickets for the dinner shows, and get your prepayment refunded.)
You can add a dining plan to a package reservation anytime prior to 72 hours before your arrival date.