Reports format (Flat OPC?), parts list, numbering availability, and revision counts. Start here.
Reads the document, writes the identical OOXML back, then compares before/after stats. Content should be untouched. (Track Changes is switched off for the write and restored after, if needed.)
Track Changes is handled automatically: the pane switches it off for the write and restores it afterwards, so results stay plain formatting. Office add-ins can't use Ctrl+Z — “Revert” undoes the last whole-document conversion for this session.
Save these and diff them offline — they tell us exactly what Word does to the XML during a round trip.
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