ADSL to NBN no outage?

Level 2

Hi Team.
I still have ADSL 2 and with the home stay we are using our data limit and will end up being shaped. I loooked to add bandwidth by changing plan (TPG does not have an assistance offer like other ISPs at this time) but only have the option to go to NBN
My concern is that if I go through the upgrade to NBN if there is no other option, is there a guarantee that this will be done without outage to the existing ADSL? We cannot afford to lose internet at this time. 
Looking forward to your reply