by Shawn Peters.
Just came across this: https://moodle.org/mod/forum/discuss.php?d=186867
I can manually select the user and change the Authentication Type to LDAP, but you can't do this as a Bulk User Action. But the above post goes through the option of either changing this through the DB via a script or doing a CSV import to 'update' the accounts.
I'll have to give it a try to see what happens.