like this kind of stuff might help
Q. If I recieve a Main Round offer, can I accept it and also change my preferences for consideration in the Late and Final Rounds?
A. Yes, UAC advises you to accept your Main Round offer. You can then also submit a change of preferences for the Late and Final Rounds. You do not jeopardise your chances of recieving another offer by accepting your Main Round offer.
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