Can a learner's course completion status expire or get overwritten?

In normal cases, no. Once a user completes a course, it will stay "completed" unless manually modified.

However, if a course is configured with a Renewal Interval, the user's completion status will reset on the date selected on the Content>Courses>Course Details page.

Note: For online courses, if you update the course with a new SCORM package, a user's completion information will not be changed.