If you are currently using ADSL, Velocity, or Cable internet, you will really have no choice but to move over. Over 18 months from when NBN becomes available the older networks are progressively decommissioned and at the 18 month mark they go into managed disconnection and will be completely deactivated.
The Gov when they first designed it added the forced migration or disconnection in to try and make sure people moved over so they could make sure everyone was on nbn and that they could ensure profitability of the network.