At UNSW, Commerce and Economics degrees both have the same 6 core subjects. If you're doing it fulltime, then that means you choose 1 'exta' per semester.
1st semester:
ECON1101 - microeconomics
ECON1202 - quantitive methods A (maths for commerce people
)
ACCT1501 - Accounting 1a
4th subject
2nd semester:
ECON1102 - Macroeconomics
ECON1203 - Quantitive Methods B
ACCT1511 - Accounting 1B
4th subject
After these 6 core subjects, everyone is free to go and pursue study in their chosen major(s).
The main difference between economics and commerce that I've noticed, is that economics people MUST choose at least one economics major to study (and they may take a commerce major as their 2nd), while commerce people are able to choose any majors they like.
As for how easy it is to transfer between economics and commerce at UNSW, I don't know. My guess is it probably wouldn't be hard.