I'm not sure I understand about changing your dates, but the way a bounceback usually works is there are specific dates covered under the offer and you are free to book a return trip for any of the dates that are covered. You don't have to come back at exactly the same time next year--the bounceback may not even apply that far out.
In terms of package, yes, my understanding is that you can convert a room only reservation, booked on bounceback to a package later on and not lose the discount.