If you recieve an offer in one round and accept it, and then recieve an offer in a later round, you can choose either to:
*keep your previous offer and not accept the new offer or;
*withdraw from your previous offer and accept your new offer.
Do not assume you will recieve an offer in the Late or Final Rounds - it will depend on whether there are enough vacancies in the course(s). Also, cut-offs may vary between rounds and could be less than or greater than or the same as those from the Main Round.