Am working on the info given ... accounts already exist ... desire to keep all users in their classes and retain their work - but switching all users to LDAP. Am I off?
In order to accomplish that 'safely', suggest the csv import of users changing only their authentication info. And setting the options for the field mappings to 'check on every login' to assure the info is correct and the Moodle consistently communicates with the LDAP server.
The syncing reads how you have configured the LDAP in Moodle. If you've configured Moodle LDAP authentication to dc=x, dc=net it will sync the entire tree - or try to. If you configure Moodle LDAP authentication to use ou=students, dc=x,dc=net then it will attempt to sync that part of the tree ONLY. If you configure Moodle LDAP authentication to use ou=teachers, ou=student, dc=x, dc=net then syncing will attempt to sync both OU's.
If you did the csv changes first, then set up the Moodle with LDAP and run the sync script, then additions/deletions would/should occur - syncing.
Uhhh, if you are setting up a test server, then there is no fear in trying what works best for you on that test server. Try things. Be bold! ;)
'spirit of sharing', Ken