Hi Naty,

Press Alt-F9 to expose the document's field codes. Every hyperlink will now look something like:
{HYPERLINK "C:\\Users\\Username\\Documents\\Some document.doc"}
You can now use Find/Replace to change the .doc strings to .docx. Do note that this doesn't change the hyperlink display text. If that's an issue, post back.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]