If you have already started doing subjects from that major, then you can't change, unless the same subjects are in your new major.
If your new major's 2005 UAI is above your UAI then you will have to apply through UAC again (or internal course transfer, not sure), but if it is below, then you are fine. Same thing applies if you are currently a new student about to enrol. If you have received an offer in say BE(SoftEng), but now want to do BE(ElecEng), then I believe you can change it when you go to enrol, but not the other way around as the UAI is higher (unless of course you meet both UAIs). This used to be the process before; I am not sure now, so don't count on this, but ask at enrolment.