As a guess....
Boost shares many of the systems as Telstra (including their billing), and that promo is specifically advertised as "New customers only". So it would be for new customers to either Boost or Telstra.
Also, if you want to move across from Boost, you cannot do it via a normal sim card. You must request (from Boost support) a special blank sim card to be posted out to you. Once you are transfered to that sim, you can then recharge on the phone/app from the available recharges options.
* I am a Boost Australia customer. This is a peer to peer community where users help each other.