Hi, my irregular offer is approved for bcom at Uom.
I called the university today, they said that the irregular offer is definite, you now have a place in melbourne uni. Your name will appear in the second round offers for the bachelor of science. Congrats! 
However this place is guranteed from now up until the final day for enrolment. I think it is safe to reject the other offer, but if you want to be 100% sure, call Melbourne uni.
I would highly recommend not rejecting your first round offer until you have accepted the irregular offer. Even though it's guaranteed, you'd rather be safe than sorry. Once you've successfully enrolled, then withdrawing from the course before the census date (31 March 2011) will be fine.