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When I was connected to NBN I was told to connect modem as soon as I received it although still on ADSL. Telstra does this as the server detects the new modem and they know you are ready to connect to NBN. In my case it worked with no problems as the new NBN capable modem also works on ADSL. The new modem automatically switched to NBN mode when transferred to NBN. In most cases the modem gets updated with your details from server but in other cases you have to do it manually.
Have you tried logging into the new. Modem going to Broadband and replacing firstname.lastname@example.org with your bigpond user ID (email@example.com) and password.