I called up and was told there's no way for an existing customer to receive the $200 credit, even after speaking to a supervisor.
It's interesting that some posting in this thread were able to get the credit, when both people I spoke to just now were adamant it is not possible!
I'm coming up on 10 years as a 2degrees customer, with two on-account mobiles as well in our house, not that that seems to count for anything...