#1043968 by bfroehle, Start API documentation for Content Migrate.

modules/content_migrate/content_migrate.api.php

+<?php
+// $Id$
+
+/**
+ * @file
+ * Documentation for content migrate API.
+ */
+
+/**
+ * Implement this hook to alter the field definition of the migrated content.
+ *
+ * Use this to tweak the conversion of field settings from the D6 style to the
+ * D7 style for specific situations not handled by basic conversion, as when
+ * field types or settings are changed.
+ *
+ * @param $field_value
+ * @param $instance_value
+ */
+function hook_content_migrate_field_alter(&$field_value, $instance_value) {
+  switch ($instance_value['widget']['module']) {
+    case 'filefield':
+      // Module names and types changed.
+      $field_value['module'] = 'file';
+      $field_value['type'] = 'file';
+      break;
+  }
+}
+
+/**
+ * Implements this hook to alter the instance definition of the migrated content.
+ *
+ * Use this to tweak the conversion of instance or widget settings from the D6
+ * style to the D7 style for specific situations not handled by basic
+ * conversion, as when formatter or widget names or settings are changed.
+ *
+ * @param $instance_value
+ * @param $field_value
+ */
+function hook_content_migrate_instance_alter(&$instance_value, $field_value) {
+  switch ($instance_value['widget']['module']) {
+    case 'text':
+      // The formatter names changed, all are prefixed with 'text_'.
+      foreach ($instance_value['display'] as $context => $settings) {
+        $instance_value['display'][$context]['type'] = 'text_'. $settings['type'];
+      }
+      break;
+  }
+}
+
+
+/**
+ * Implement this hook to alter individual data records as they are migrated.
+ *
+ * This hook is called after the old data record has been read from the
+ * database and before it is inserted into the corresponding D7 field. The data
+ * column names are renamed in a one-to-one mapping by the order they appear in
+ * the database. This is often the desired behavior, but in some cases an
+ * implementation of this hook may need to move data between columns.
+ *
+ * @param $record
+ *  The data record, as read by _content_migrate_batch_process_migrate_data().
+ *  If the ordering of the D6 and D7 field columns remain the same, no action
+ *  is required. If the columns were re-ordered or the data format was changed,
+ *  $record should be modified to fit the new field definition.
+ * @param $field
+ *
+ */
+function hook_content_migrate_data_record_alter(&$record, $field) {
+  switch($field['type']) {
+    case 'file':
+      // Map D6 filefield field columns to D7 file field columns. Note the data
+      // which was previously in the 'data' column is read into the
+      // 'description' column since the data column no longer exists.
+      if (!empty($record[$field['field_name'] . '_description']) && ($data = unserialize($record[$field['field_name'] . '_description']))) {
+        $record[$field['field_name'] . '_description'] = $data['description'];
+      }
+      else {
+        unset($record[$field['field_name'] . '_description']);
+      }
+      break;
+  }
+}