From what we understand with the new NookPress, if we want to make any changes to an existing book, we have to unpublish it, download the epub from that old one, add the changes and then make a new project. What happens to the unpublished one? Haven't a clue.
And Congratulations on publishing your book!!
I ended up making a new book out of my old one today, to be able to improve my manuscript. I knew no other way and could not get any info on the "chat" feature.