There is no way to force the storage driver to write the file in another version (6), but there is a way to force retrieval driver to read newer files. For that you need to inherit BinLDrivers_DocumentRetrievalDriver and redefine the method CheckDocumentVersion. Of course, you also need to make and register your own document plugin. The application that uses old OCC should be patched then.
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Backward XBF Storage Compatibility
Starting from OCC 6.7.0 the BinLDrivers were upgraded to version 7. What makes impossible to open the new files with the older versions of OCC (6.6.0 for example). Is there a way to force the storage driver to write the file in another version? (version 6)
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Please note that this is offtopic for modeling theme. It would be better to create a new theme "Application Framework" for this topic.
As a follow up - the method TDocStd_Document::ChangeStorageFormatVersion() has been introduced with limited support of writing documents in older format version:
https://tracker.dev.opencascade.org/view.php?id=29217