Simply contact Boost using their live chat function, which is available at http://livechat.boost.com.au/BOOSTorting. They are available 24/7. They will be able to assist you with transferring your number.
If chatting online isn't your thing, you can contact Boost by calling 125 8881 directly from your new Boost SIM. This number will work even though your SIM hasn't been activated.
It can take up to 48 hours for your number to transfer.