If you only missed out via ATAR for Commerce, I'd recommend keeping that at Preference #1 and putting similar courses as lower preference. If you've missed out on Commerce for other reasons e.g. didn't do Methods, then I'd be a bit more hesitant and considering pathways more seriously. (It depends on your situation a bit.)
is it possible for me to change courses or change course
Yes, certainly.
what would i need to do
So you've got the following options:
1. bridging course. If you just want to do the Bachelor of Commerce at Monash or Melbourne (and not considering any other courses), you can try a bridging course. This is usually for those who missed a subject prereq, but some others do it simply because they missed out on a spot in previous years. You'd definitely have to ask around to see if this option is available though, as this is only applicable to some courses and I honestly don't know much about these. I would recommend contacting both unis to see what options they have.
2. internal transfer. If you start off by studying a Bachelor of Business or similar, get competitive grades (usually a 70+ WAM, 75+ to get in comfortably), you can usually attempt transfer and switch out to the Bachelor of Commerce at the same uni after a semester or two. To apply for transfer, you'd usually have to ask the course enquiries when you'd apply and/ or watch your uni emails, typically around the halfway mark or latter half of semester ad start the application process.
3. external transfer. Same as internal transfer, except you're going from one course at one institute to another course at another institute. Make sure to ask about course transfers before starting and when you're applying for transfer though.
4. There's probably some other options, but I've gone blank. I would probably contact the unis to see whether I've missed anything.
\and how long would it take? thanks guys
Depends on your route, but the quickest route is usually a semester.