It depends. If you have a package & book a bounceback package, they can probably modify your existing g reservation & apply your deposit. But if you go from room only to package or vice verse, they might need to make a new reservation. Room only & packages have different deposit requirements, so you would need to meet the new deposit requirements too, no matter if they rebook or just modify.
Also, having a reservation already will not help you get a bounceback rate. If there is availability at the resort & room type for the dates you want when you call, you can get the rate. If there is no availability, having a reservation already booked won’t get you the discount.
If you want to make a future reservation so you have the resort & room type for the dates you want, book whenever you’re ready. If you can only go if you get a bounceback rate, might as well wait til you get there & see what’s available at the time.