He does it a lot it seems lol
From one relative newbie to another you can defiantly un-publish to add things and expect to do so unless you have a proof reader, I was impatient and didn't wait to proof read mine so I had my first story published for about 24 hours, got maybe 5 reviews then pulled it again to fix, spelling, grammar and other minor errors.
This lead to me being annoyed with it. A couple of months on and I have doubled the story in size (page count) added loads to existing branches and added entirely different endings... I will be re-publishing it just as soon as I can write the final branch and trawl through the hundreds of pages to check for mistakes now that I altered it all...