by Ken Task.
I think the only way one could keep existing users, their enrollments, assignments, etc. is to update their accounts via csv changing only their auth. In the mappings of LDAP -> Moodle set the key fields to update on every login for a while until all account information in Moodle matches that of LDAP. Of course that assumes that the username users use to log into Moodle can be found in LDAP.
'spirit of sharing', Ken