If you wonder how to update an already-shared document to your iWork.com account, it's somewhat hidden in the interface. The Share menu will keep saying Share via iWork.com. However, if you do it like this, iWork.com will replace the document, but keep the comments intact:

1. Share the first version, add contacts and a title.
2. Edit some content ... or don't.
3. Share the document again, making sure you select the Title from before in the "Copy to iWork.com as:" drop-down.

If you do this first, you will see the former contacts filled in again, and you can then customize the Subject and Message. Hint: Open the online version while doing step three, and see how iWork.com auto-fetches the update in the browser.