The way I'll be implementing this is:
Just before the report is generated, check if a submission.xml file exists.
If it does,
prompt the user if they want to keep it.
If they do,
Rename the old submission to include a timestamp of when it was create
Write the new one to the default location
Perhaps some sort of cleanup will need to be done to, to make sure there aren't thousands of submission.xml's floating around (seeing as the user has to explicitly request to keep the old one, this may not end up being a problem)