OP, in my opinion I think it's definitely achievable to transfer to a Law double degree at the end of first year. Year 12 is very competitive and you have a lot of people interested in that course and its difficult to achieve the 98+. If you commence 2015 at Monash and work hard to try and get a good WAM, I think that your in good contention.
I also think an 80+ WAM is slightly more easily obtained than a 98 atar. Although this means being on top of Uni work from day 1. Start working on assignments well before the due date, organize a good balance between assignments/private study, know your lecture material back to front. You start the year with a potential 100 marks, you should have the mindset that your going to be keeping a lot of those marks. A friend of mine and myself, have managed to complete first year with a 80+ WAM, he is looking to transfer courses (similiar situation as yourself but a year ahead).