by Richard Schwarz.
Hi,
I know others have already posted something like this here, but this is what I did in PHPMyAdmin to bulk change them (and leave my guest and admin accounts alone).
Run in PHPMyAdmin, or some other SQLinterface.
UPDATE mdl_user SET auth='ldap' WHERE auth='manual' AND id>2;
This updates the user table, changing the authentication method from Manual to LDAP, except for Guest and Admin accounts (the first two created in a Moodle install).
It's a good idea to have your admin account set to manual, so that if LDAP falls over (or your admin account doesn't exist in the Active Directory (or whatever you're LDAPing to), you can still log in. Also, most people won't have a guest account created on their AD Server, so keep it alone too.