Converting Drupal user to LDAP user

I have a user that appears to have a native Drupal account (on his user page I can change his password). My other users are LDAP users (on their user pages I cannot change their passwords). The user in question is using the same username as his LDAP account but a different password, so I know it's not using LDAP for authentication.

How do I convert this user from a Drupal-authenticated account to an LDAP-authenticated account? The only thing I can think of would be to delete his account and then have him attempt to sign in using his LDAP credentials, but that's a last resort.

Thanks.