I'm not sure if I can get an atar high ecnough to get me into commerce at UNSW, but say I get an atar which gets me into eco, is it possible for me to transfer to commerce within 1 semester (which I heard is approx 14 weeks) ?
No, you won't meet the Minimum UOC requirement (36 UOC). You would only have completed 24, maybe 27 or 30 if you overloaded (which I doubt you'd be allowed in first semester of first year).
Anyway, just the courses that are common in both degrees; if you are adamant you want to do commerce, just do the commerce subjects, and transfer at the end of the year. Risky move, but it should be fine if you work hard to satisfy the WAM requirement.