What happens to the data in current LDAP,if I rebuild it and install it again?
Most likely it will still be in the database and openldap will still use it. For me that is at /var/lib/openldap-data This is unless you are deleting the old system with your reinstall.
You probably want to make a backup of your data.
slapcat > backup.ldif
Should I create all the users again?
No. If you did not change db versions or wipe your disk the data should be there. If you did make a major change in db version you it probably will find it better to restore the backup.
slapadd -l backup.ldif
John
John