The lowest-risk migration: keep the classic theme, add theme.json, and start using block patterns and blocks alongside existing PHP templates. The site stays the same; new content gets block-editor-friendly authoring.
For a full block theme, the key files are templates/index.html, parts/header.html, and parts/footer.html — all HTML, no PHP.
Tools like Create Block Theme export an existing block theme back to theme.json and templates after edits in the Site Editor.