Can I turn a book I already have into a collection of stories?

Yes, from version 1.9.20. Open the book's page, then Book details in the right column, and tick "A collection: every chapter is its own story". The settings of a collection appear under it: what the stories are, what ties them together, what every story shares, and whether exports show only the story titles. It applies to everything written from then on: new stories added with "Add stories" and chapters you regenerate are written as standalone stories, different from the ones the book has. Chapters already written stay as they are until you regenerate them. Untick the box and the book is one continuous book again.