There are 2 ways to allow students to access past courses:
Go to Users and Groups in the Control Panel of the course and select Users
Select Username and Not Blank in the Search, then click Go to see all the enrollments
If you cannot see the user's name on the page, type in last name or username of the user you wish to modify in the Search box, type the appropriate search term, and click the Go button
Select the context menu button (down-facing arrow) next to the user you wish to modify
When the menu pops up, select Change User's Availability in Course
Choose the desired availability option for this user (Yes/No)
Select the Submit button at the bottom right of the page.