There are deadlines, and you'll usually have to do the course for the remainder of the semester. Transferring is a fairly simple process, you simply go to the Student Centre (contrary to popular belief, they are better than Centrelink) and ask to apply to transfer. You'll be given a fairly simple form to fill out and you have to attach a letter requesting the transfer and submit it and they'll inform you of the decision with the new enrolments; if you apply in first semester, you'll have to wait to second to be able to transfer.
That said, you'd probably have a better chance of getting into (hypothetically) Arts/Science if you were doing Teaching/Science (I pretty damn sure it's possible) as opposed to a plain vanilla Teaching since you'll only be changing half your degree as opposed to the whole thing. For example, it's easier for me to go from Business/Economics to Business/Arts and major in Communication Studies than it is to go from my current course straight to Communication Studies itself.