You will almost certainly be required to at least do a bridging course, if not year 12 chemistry. Bridging courses are available through some universities, also through open universities (check their website) and they may be offered at some TAFEs as well. You should try to sort that out as soon as possible, and call the university to see what they would require and if there is anything else you should do.
As for changing courses: if it's within your university, you can apply to transfer. But, if you're looking to go to a different university, you will either apply to VTAC or (specifically if you want mid-year entry) to the university itself. Make sure you check deadlines and so on.
Because you won't be a year 12 applicant, admissions will look at your results overall. This includes your ENTER as well as the scores you've obtained in Urban Planning. Apparently they can be flexible. Someone I know ended up doing Arts in university but then decided that she wanted to do Nursing. Her Arts grades were not very good because she chose the wrong degree for her interests. However, because her ENTER was high enough to get into Nursing in the first place, she was able to transfer. Likewise, if your ENTER isn't very good, that's okay, they're likely to put more weight on your university scores as those are the most recent indicator of your academic performance.