they cannot force you to go casual (although these stupid IR laws may somehow allow it), and you should refuse to do so if they push the issue (maybe tell them that you need to see whether your laywer thinks that asking you constitutes discrimination!).

I am a casual employee (I always have been in this position and am lucky because I am NEEDED, and the organisation is very flexible for me). As a casual however, I am not entitled to maternity, sick or any other type of leave. If I cann't show up I don't get paid (If I didn't show up too often then I would likely get sacked). Basically as a casual you have little to no rights as an employee - there is no such thing as job security.