@Techaddict Yes, you will have 18 months after the NBN is active in your area to find a provider.
However as you are on Cable your area may be remaining with Cable, which will be called HFC (Hybrid Fibre Coaxial), and Telstra are one of the RSPs (Retail Service Providers) of the NBN. Your current Cable Contract may even include the swap over to NBN as part of it's conditions.
If at all concerned contact your local Telstra Store or ring Telstra to discuss what will happen. You will get a new Modem once the NBN is in place but that should be about the only change you will notice.