Joining the NDIS puts you in charge of that question.
If you're happy with your current supports and they meet the reasonable and necessary test, you can keep them. Your plan can fund the same providers and services you already use (and if you're plan-managed or self-managed, that includes providers who aren't NDIS-registered).
At the same time, the NDIS usually opens up more than people had before: a wider range of supports, more providers and funding tied to your goals rather than to whatever programs happened to exist locally. So it's worth treating your first plan as a chance to review what's working, keep the good and explore what else could help.
Mainstream services (health, education, Centrelink) continue alongside your NDIS plan. The NDIS doesn't replace them.
Building your first plan and want to make sure nothing you value falls through the cracks? Email hello@providerchoice.com.au.