Greetings! Yes, you can change the due and availability date on assignments and not lose student submissions or grades. The one caveat is that if you change who the assignment (or discussion or quiz) is "Assigned to" (and remove people who have already submitted) then it might look like you lost submissions and grades. Yet, as long as you add everyone back to the assignment, then everything will pop back up. 🙂
In terms of how can you lose assignments or grades.... In general, it's pretty hard and doesn't happen very often. Discussion responses can be deleted by faculty and those can't be gotten back. Yet, in my almost 10 years being a Canvas admin, I'm not sure if I've ever had this happen to an Instructor. Assignment and quiz submissions can't be deleted by Instructors, so you're good to go there.
In terms of grades being lost or changed, grade history seems to work pretty well - How do I view the history of all grading changes in the Gradebook?
Hope this helps! Kona
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