When a student’s schedule changes (Courses/Sections changed in Schoology), the student will not be able to access the assignments they completed in the previous course/section. Assignments are tied to the course/section, not the student. Students should save any desired materials before a class schedule is changed. It is difficult to retrieve assignments/materials after the change is complete.
Here is a document explaining how to save course materials from one course so they have access to them once they are in their new course.