Yes, you can accept a new offer and withdraw from your first one.
I don't think that changing prefs now changes your first offer..
UAC have already posted out to you your offer anyway.
Once you get an offer, its solid.
Was BPharm @ USyd her first preference?
Maybe her STAT results weren't properly attached to her application or something.
Give the Pharmacy faculty a call tomorrow morning.
Re: offers came in the mail already?
I think what's happened is, that UAC probably instructed Aust Post to make sure they arrive tomorrow, but maybe by mistake, Aust Post delivered them to some Postcodes today. Probably the mistake of mail sorters..