Server downtime is minimized. At any given time, most servers are running and available.
Most of the LDAP directory is available to the servers and for provisioning.
You migrate the LDAP directory in stages, selecting individual domains for migration.
The overall migration time is extended.
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.
This migration process is the most complex of the scenarios; it is more complex than a direct migration to Schema 2, native mode, or an incremental migration to native mode. The schema migration must be performed twice.
For a discussion of the conditions best suited for migrating your installation incrementally, see Deployments Suitable for Incremental Migration.