If you haven't ordered an NBN service from Telstra, then you won't get switched across. It's not an automatic process.
I'm still not sure why they are dispatching new modems when people haven't ordered them. There is something slightly odd going on.
Anyway, you can connect the new modem if you want, but you will need to enter in your currect User ID and Password that is in your existing modem. An ADSL connection requires these details. An NBN connection doesn't and the account verification is done within the network itself.