FWIW, this is what I was told on my last cruise in November. By the way, the Cruise Next staff were very helpful, even though I did not book a cruise or buy any Cruise Next certs.
I had the manager look up a few cruises for me. I asked him , if I booked a cruise, who would I call to ask questions or make changes. He said the booking credit goes back to how you booked the current cruise. If you used a local travel agent or NCL PCC, they get the booking.