The older version will convert the IDML back into an INDD file automatically when you save it. You will lose some new features (like certain SVG effects or font technologies), but your layout, links, and text will remain intact.
Always keep the original new-version file. The IDML file is not a live link; it is a snapshot. Any edits made in the old version cannot be automatically synced back to the new version. how to convert indesign file to older version link
You must first open the file in and export it as an InDesign Interchange (INX) file. Advanced & Third-Party Options The older version will convert the IDML back
after opening the IDML in the older version to create a new .indd file. but your layout