And just to add to the confusion...
I just called NCL and ask the same question... The agent this time said I would not be able to use the certificat, not even the amount I put in, 150$. The certificat (300$) would go back to my Latitude account and stay there until I book a completely "new" cruise. I would have to give new $$ as deposit for rebooking the cancelled cruise... She said "her" understanding is that "additional value" is the certificat itself!
Why can't all NCL agents speak the same ????
Anyway, the promotion has ended, and I still have my original booking. I now just hope that if there is another price drop, it will allow to "reprice", so I can benefit of a lower price.
But I know for sure that I will not buy another CF to use on a cruise too far from final payment.