it won't change your marks, you'll keep all your marks, it's just that your UAI contribution would be less. See if you got 95 in your HSC for cafs and got 95 for Engilsh advanced, English will take your UAI sky rocketing but cafs won't have that strong of an impact.
This knowledge sent me into complete panic for the whole of my HSC year and I fought tooth and nail to make sure I ranked first as well as ensuring I knew the syllabus and content inside out and back to front. It shouldn't be a problem if you know that you can rely on yourself to get through the course, unlike in some other subjects you probably can't fall back on the others in your class because they probably don't care as much as you do (no one else in my class cared, except maybe one or two) also try and send out your notes and essays to other people in your class after the trials are over and tell them all to memorise, so that if you do stuff up the exam you have some slim hope of someone else holding you up.
I ended up coming first by about 4 marks, which benefited me in the HSC so much because the girl who came 2nd ended up with a final mark of 89 and anything under 90 WILL unfortunately, have a negative effect on your UAI.
Don't stress out too much (I know I made it sound quite doom and gloom) just trust in yourself and whoop everyone's arses!