The general course is different, it might be very hard to catch up, it is probably better for your UAI to work harder at adv maths and make sure you understand it, it wont take more time then trying to learn the whole prelim General Maths
I wouldn't suggest you to change your subject now as it is a bit late in my idea. General and adv math are two completely different things.
It's never too late to catch up. I got like band 5 in my School Certificate and no one in my school think I can do well in 3 unit math.(in fact the teacher even suggest me not to take 3 unit math) But now I am coming first in my math class just because I really tried to catch up in year 11. So just try you best to catch up and I am sure you can do well in your HSC.