If your are in year 12 absolutely --> don't do it dude... too late! Generally people drop down from advance to mathematics to general for a reason, it takes time. The mathematic's course 2unit to 4unit is known as the "Calculus course" and yes if your going to a half decent school they would have been way into Calculus by now. I wouldn't even recommend it if you were doing the HSC all over again unless you take an extra year or 6 months to do the prelim course. In fact a a 2-4 unit maths teacher would have difficulty helping a general math student and like wise a general maths teacher will have difficulty in helping a 2-4unit student because they are teaching very different courses that require prolonged practice and a natural feel acquired provided by such practice.
The difference is more like Economics to Business Studies, both the same ball park but and very different content! A great teacher will tell you Year 11 is the time to experiment, it's sad to say that you have exhausted your opportunity to see how well you would perform in 2 unit mathematics. Keep in mind that 20% of the HSC test can be tested from the Preliminary syllabus, since that 20% cannot in anyway be pointed out by the Board of Studies you are pretty much being tested through the entire Preliminary and HSC syllabus. Anyways staying in general gives you the advantage of having already learnt most of the content in Year 10 since you were in 5.3. The HSC general math course can be ridiculously easy that it's repetitive it's almost like doing a more structured school certificate maths exam. If your good at math your guaranteeing your self a lot of free time specially towards exam time if you do general. As long as you can guarantee that you can get a Band 6 and a good chance of getting 95 to 100% your negating scaling advantages (turn over marks etc, looking at the distribution of marks blah bah and seeing a turn over point). A band 6 and 8 units of band 5s is 90+ ATAR <--
If your are in year 10 or 11 absolutely --> Do it yo! Advance Mathematics and Extension One Mathematics is is one of the most rewarding experiences i've had, chances are 2-4 units maths is one of the fairest courses in the HSC hands down. People cannot wing these courses, so its the best memorisers and students who've done a ridiculous amount of past papers that top the state. Honestly this kid in my school was winging history, religion and english not reading books, doing work and etc and averaged 90%+. Does this mean difficulty is ridiculous? My experience was that through more abstract math i became more appreciating of mathematics all together (i was going to say "mathematics in genera"l wanted to avoid the pun) with that it became less tedious and more interesting i wasn't the only one, mathematics seems to be a good topic for conversation however weird that may sound, you can even tell that a lot of the teachers miss it. Have you ever googled/wikipedia the stories of Ramanujan or Pascal/Fermat? What i am trying to get at is that its just more fun/rewarding, not so much in prelim where all the unmotivated people who would go soon on drop to general are still in the room, specially in the HSC where it is very competitive and the content is very interesting. If you do things right at the end you look back thinking "wow i can do this". I struggled through most of the course MX1 and 2U, for reason out side my control the stars jus weren't aligned, but hey i still edged out a Band 5 and an E3 i got like (two other Band 5s) which i think got me the maximum 5 academic points for the courses i wanted in it's self. While your at it give MX1 a good kick in the ass and see how you go. As long as you study hard and study smart (use appropriate memorisation techniques) you might have just made the decision to take the courses that MADE your ATAR. At that Extension 2 mathematics is definitely the absolute EASIEST and most POPULAR way to get 99.95 (not everyone's goal but you get the point... High). Doing 3 Unit work was one of the best part of my day, it might be yours too!
But then if you worried about the work needed and don't feel passionate about maths ever and you know you won't change, stick to general. Do not take courses out of scaling or prerequisites most Uni courses do not use perquisites that much just recommendations although i'm sure as hell don't know how people with no maths at all can bridge course their way past MX1 and MX2 contents the calculus course is way to hard to catch up in 3 days when you factor in binomials, cartesian equations, even geometry of the derivative. Also you need to ask your self now what course you actually want and pick your as up and look it up. You should look at how your HSC courses can add up bonus points in different bands.
