To convert an existing FlexBook to the new 2.0 format, navigate to your CK-12 Library. Click the "Create New" button, then "FlexBook 2.0"
Click the "Add Content" button, and choose "Search CK-12."
Enter the title of the FlexBook you want to convert, then "Library" to filter results from your CK-12 Library. When results are returned, click the title of the book you want to convert, then click the "Add to FlexBook Textbook" button.
You should now see your existing book in the new 2.0 format.