While the directory is in Schema 2, compatibility mode:
User-developed applications can continue to use the LDAP directory exactly as if it were still in Schema 1.
Messaging and Calendar servers can continue to use the directory exactly as if it were Schema 1.
User-developed provisioning tools that rely on Schema 1 can only work on existing directory data.
The process is more complex than it is with a direct migration to Schema 2, native mode. The schema migration must be performed twice.