Hi @MacMag Welcome to the community.
Your existing services need to be disconnected first in order for the new connection to work, that's assuming you're going from ADSL and a normal PSTN phone line to FTTN NBN, what would normally happen on installation day is that the phone and ADSL will stop working, if you monitor the status lights on your current modem you will see when this happens, at that point you should plug in your new NBN modem (which would normally arrive a couple to a few days prior to your installation date).
It may take up to a few hours for the new service to be active, possibly sooner. You don't need to change any settings as the new modem will auto configure itself on the network. You need to allow up to a few days for any number porting to be completed but can happen much sooner in some cases. Keep an eye on your email inbox as this will have updates.
