yeah different combined arts degrees have different entry cutoffs
they convert your uni results (HD, D, C, P) into a mark and average that with your UAI
usually you'll have to apply through UAC like you did after the HSC. you won't get a new UAI you'll just find out along with the year12s whether you get in or not
some courses have mid-year entries, but from memory i don't think arts/commerce is one of them. if you're already doing a similar course try talking to the arts faculty or something.