For high-volume production environments or agencies that frequently handle legacy workflows without active CC subscriptions, third-party software tools can automate this process. Markzware offers a popular tool called IDMarkz, a standalone application that previews and converts newer InDesign files into IDML formats without requiring an active Adobe Creative Cloud subscription.
in the older version of InDesign. It will open as an "Untitled" document, which you must then "Save As" a standard .indd file. Alternative Methods
If you need to fix a specific error during your conversion process, tell me: