This is different from what my experience is. I called on Friday and spoke with an NCL rep and he was able to use my Cruise Next certificates to book me on the Spirit for August of this year. Fortunately I was able to take the offer of the 50% discount for this summers sailing. I thought after reading the cancellation letter that I would have to wait to get to use the certificates, but he was able to process them on Monday with no problem.
Sorry that you are having such issues with trying to rebook.