There’s a ChangeLog describing the API-changes: Changelog — LVGL documentation
We’re not aware of any v8->v9 converters, but SquareLine Studio v2 will be able to export LVGL-v9 code, until that I guess you can ask if a converter exists or is planned, in LVGL forum or GitHub-discussion, e.g.: Changes in master (v9 development) · Issue #4011 · lvgl/lvgl · GitHub , v9 discussion and changes · Issue #3298 · lvgl/lvgl · GitHub