So once you accept the offer you are enrolled in the course? ALSO how long does it generally take to accept the offer and confirm all the details?
Can you still change your preferences after you receive your ATAR or would it be too late because on the UAC site it says 'Applications close for consideration in Main Round' on Fri 13 Dec 2013 but ATARs come out at 18th of Dec?
It also says on the UAC website that applications close until July but most courses start in March.....WTF?
In terms of accepting offers, it's a bit more strung out than that. You accept the offer and either it will direct you to enrolment on the university's website or it will give you a date where you can enrol in person. This dpeends on the university. In terms of accepting the offer, it's pretty much instantaneous.
Yes, you can change your preferences after your ATAR is released BUT the changes you make by that stage will not count for the Main Round. In the end, it shouldn't matter because as long as you preference the degrees in the order you would like to do them, then you're not wasting anything.
Applications close in July because UAC need all the data in advance to set-up the system which allocates all the spots. It isn't something you can do immediately.