What exactly does "Restyle" change in a chapter, and what does it keep?

Restyle (version 1.9.19 and later) is a rewrite that changes how the chapter speaks, not what it says. The AI is told to keep exactly every fact, number, name, quote, example and argument, the order of the sections, every heading, the paragraphs' places, images, tables and the layout marks, and the chapter's length within about ten percent, and to keep word for word any sentence you edited by hand that already fits the new settings. It changes only word choice, sentence rhythm, the way the reader is addressed and the register. A rewrite that comes back much shorter than the chapter is refused and the chapter is left untouched. Before anything is rewritten, the current text is saved in Versions (in the chapter editor), so you can go back. A language change cannot be restyled: that is a translation, so the card offers Regenerate, or you can make an edition from the Editions card. A title or subtitle change cannot be restyled either: that is the book's promise, and the Promise card checks whether the chapters keep it.