System Administrators can change a user's Org Unit access at any time. 

Changing Org Unit Access

Navigate to Settings - User Management. Find the user in question, and click the pencil icon to edit their profile. Changing Home Org Unit will change the user's default Org Unit. Any additional Org Units the user needs to access can be added or removed from the Additional Org Unit dropdown menu. Note that a user will have access to any sub-units that sit under an Org Unit or Legal Entity they can access.