Closed Digital Edition.
Rename the Folder My Digital Edition under you Documents to My Digital Edition.old.
Start Digital Edition.
Go to File > Add to Library.
partly correct - now I have two folders of ebooks, the new one contains the new book only. Adding the books from the "old"-folder is leading to a hang of programm... sorry, I don't need this poor program for managing my books, I only have to use it because of the DRM. I would highly prefer a slim app to manage the DRM only. It would be perfect as an addon for Calibre...
I have only try to help.