Hi - generally this error is because the main data source was changed in the form template. It sounds like you have added and deleted fields.
There are versioning settings - you can set the template to upgrade existing XML. If you've deleted, moved or renamed nodes in the main data source of the template, you will have data loss if you upgrade your existing XML.
You can test your current template with existing XML to see in a non-destructive fashion what you need to do to resolve this - here's a blog post of mine on previewing your template with existing XML.
However, if the user opens the current template, and creates and saves a file, if the template hasn't changed, they should be able to reopen that XML without an issue - is that the case? Can you create, save and close a form, and then immediately re-open it and not have an error?