I spoke to my english teacher today (she's an hsc marker and is on the advice line and everything) and she said that:
1) They CAN'T specify texts to write about from the Stimulus Booklet, because some people are doing Changing Self, others Changing Perspectives and Changing Worlds. Also if you have a look on the first page of the Booklet it says something about choosing texts.. i can't remember what exactly it says but it means they can't ask you a particular one.
2) That the BOS CAN (it doesn't mean they will) specify texts from your Prescribed Text for change, eg. for me its Gwen Harwood. That means they could say, something about change yada yada, refering to "Mother who gave me life" and "The Glass Jar" and the Stimulus Booklet and two other pieces of related material etc.
or, if they are nice, they could say "refer to 2 of your poems for prescribed text, the stimulus booklet and related material" so you can choose.
Hope that made sense.