Can a relationship between two characters change during the book?

Yes, from version 1.9.22. In the Codex, under Relations, open the relationship with the arrow at the end of its row and find "How it changes along the book". Press "Add a change", enter the chapter where it changes and what the tie becomes (for example, from chapter 7 the friends become rivals), and how each of them now feels. Add as many changes as the story needs, then press Save. The AI writes the chapters before the change with the old relationship and the chapters from it with the new one, so a friendship can turn into love, or a partnership into betrayal, exactly where you planned it.