pure stupidity
the two have absolutely nothing in common, all of the subjects you do in the first year wont count at all. If you are going to try and transfer you start with something that is similar to what you want.
Also, I am pretty sure you will fail science/commerce subjects anyway because you failed to get a high enough atar to get in straight away, suggesting you most likely didnt put much effort in for your hsc .
science/commerce is not that easy!!, and arts/law on the other hand is the biggest bludge ever