If you have the ATAR for B. Science, I think an internal transfer shouldn't be too much of a hassle.
If you don't have the ATAR, then keep your WAM reasonable (I have no idea how high you need, but try to avoid dropping below credit average; the higher the better) and transfer into B. Science at the end of this year after you get your results. I'm pretty sure that most of the units you do in B. Liberal Arts and Science will be transferred as a part of B. Science later, so I wouldn't worry too much about that.
You can try emailing the science faculty to get a better idea.