Pardon the exclamations but we're creative folks not techies. After editing a CS3 file in CS6 we are unable to open it in CS3. Looks like CS6 will only save back to CS4? So once edited in CS6 then I can no longer use CS3 to work with the files? Can anybody confirm this for me? We also tried to export to IDML but CS3 would not read it.
When you open an older version of InDesign file within a newer version and edit them, the older version of file is first converted using some required plugins of the newer version of InDesign and then a consolidated new file is generated including the required plugins. Now, if you try to open this newer version of file back in an older version, the older version of InDesign looks for the required plugins used to edit the file and fails because InDesign older version did not use those plugins to create your file.
Please let me know if that explanation helped understanding why this behavior occurs.