Without looking up the format, I have to say that .DOCXfiles appear to be a cinch to edit. I just unzipped one, changed into the worddirectory it created, and popped open document.xml.

While it has the typical Microsoft Word overabundant nonlinear code insertion (I did hack one of the early .DOCformats way back when and remember that observation being applicable), the raw text components seem to be fairly consistently in between <w:t>and </w:t>tags. Before writing production code I'd want to research that a bit, of course, but for a quick-n-dirty word replacement I'd probably be willing to gamble.

Sample of what I found:

<w:t xml:space="preserve"> so we can order food.  Please 
include in your RSVP how many people will be attending.
</w:t>
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