If all the servers in your installation manage across domain boundaries (if multiple servers share access to each domain), your installation might not be a good candidate for incremental migration.
For example, suppose your installation contains two back-end servers in the following configuration:
Back-end Server 1 manages Domain 1, 2, and 3.
Back-end Server 2 manages Domain 2, 3, and 4.
This installation should migrate directly (both servers, the entire LDAP directory), not incrementally.