It's going to be tricky asking a QLD related question on the NSW forums, but here I think we have a last minute chance to change our preferences after we get our ATARs (equivalent of OP) before they are locked in and offers are released. If it's the same in QLD then I suppose you can change it if you're not feeling confident by having a safety net. Otherwise you'll have to look into alternative pathways into your desired course at uni.