A package requires a $200 deposit, with balance due 30 days prior to arrival. Changes can be made without penalty up until that 30-day point, and may be charged a $50 change fee if within the 30-day window. Tickets with a package are valid from check-in through check-out, or length of ticket, whichever is longer. You should not lose theme park reservations by changing your package - but ability to change check-in/check-out date will depend on resort room availability and may not be easy to change.
Tickets purchase separately must be paid-in-full at the time of purchase. You will need to set a "start date" and that determines your ticket price, and also determined the valid use window along with number of days. A room-only reservation requires a deposit equal to 1-night stay, with balance due at check-in (or charged to your card that morning); cancellation with no penalty is allowed up until 7 days prior to arrival. Changes to check-in/check-out date will depend on resort room availability and may not be easy to change.