Yeap, that thread was useful for getting the message box, but I'm missing all the checking conditions; i.e., if the filename of the record in the Sharepoint list already exists.
Does submit use the "Save" methods? I thought those were restricted to save operations.
On thinking through the requirements further, here's what I would like to happen:
1) On Submit, if the date is the same as it was when the form was first filled out, then present a "Do you want to overwrite?" message box. If they say no, the user needs to change the date.
How do I establish whether that date has changed? The IsDirty property seems to apply to the XDocument object, not a specific field within the document.
Thanks!