Check whether the document has the 'Automatically update document styles' property checked, under Developer>Templates. If so, the simplest solution is to change the font settings in the Style definitions in the template. Although you could uncheck the 'Automatically update document styles' setting, that's not going to help for a document that's already been compromised. Indeed, even if the 'Automatically update document styles' setting isn't checked, doing so and changing the font settings in the Style definitions in the template may resolve the problem.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]