Ctrl+A, Ctrl+Q, Ctrl+Spacebar will return all text to the paragraph and font settings prescribed by the corresponding Paragraph Styles. Do note, though, that this also removes all Character Style formatting. However, having done that, you can save this as a copy of the original document, then use Word's document compare tools to find the differences at the Paragraph Style level.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]