Changing a User's Manager

I have a very large set of users, approximately 900.  I would like to update the User's Manager.  So far I haven't been able to export the users and re-import them to change the manager.  Is there a way to do this?  Or am I stuck updating the manager for the users one at a time?  It's not feasible to have to manually update each user.

Has anyone been able to update the Manager field by doing a re-import?