How do I migrate only the items that previously failed? I have a migration class that has imported 2000 items correctly, but failed to import 40 of them. 

Now I have corrected the import class, but I don't want to do a rollback and a full reimport/migrate.

Is there a possibility to import/migrate only the 40 failed items?
Another possibility is to use the "--needs-update" option as "migrate-import Article --needs-update" . 

You might need to status of needs-update field for these 40 records as needs_update=1. 

This option is only needed when your Drupal DB is on a different DB server from your source data. Otherwise, these records get migrated with just migrate-import.