Go to Site Administration -> Users -> Accounts -> Browse list of users -> click the cog next to a user with a manual account to begin editing their profile ->change 'Choose an authentication method' from manual to LDAP -> click 'Update profile' button.

The record will not save because the username field is disabled and yet required.

As a temporary solution I found by removing the 'disabled="disabled"' attribute in the input tag allows the record to be saved. (See attached image)