Only in feeds.a/mappers: field.inc
diff -rup feeds.b/plugins/FeedsNodeProcessor.inc feeds.a/plugins/FeedsNodeProcessor.inc
--- feeds.b/plugins/FeedsNodeProcessor.inc	2012-10-11 21:46:10.000000000 -0400
+++ feeds.a/plugins/FeedsNodeProcessor.inc	2012-10-31 01:38:13.725628000 -0400
@@ -50,7 +50,8 @@ class FeedsNodeProcessor extends FeedsPr
    * @todo Reevaluate the use of node_object_prepare().
    */
   protected function entityLoad(FeedsSource $source, $nid) {
-    if ($this->config['update_existing'] == FEEDS_UPDATE_EXISTING) {
+    if ($this->config['update_existing'] == FEEDS_UPDATE_EXISTING ||
+        $this->config('update_existing') == FEEDS_SKIP_NEW) {
       $node = node_load($nid, NULL, TRUE);
     }
     else {
@@ -68,7 +69,8 @@ class FeedsNodeProcessor extends FeedsPr
     }
 
     // Populate properties that are set by node_object_prepare().
-    if ($this->config['update_existing'] == FEEDS_UPDATE_EXISTING) {
+    if ($this->config['update_existing'] == FEEDS_UPDATE_EXISTING ||
+        $this->config('update_existing') == FEEDS_SKIP_NEW) {
       $node->log = 'Updated by FeedsNodeProcessor';
     }
     else {
@@ -214,6 +216,7 @@ class FeedsNodeProcessor extends FeedsPr
       FEEDS_SKIP_EXISTING => 'Do not update existing nodes',
       FEEDS_REPLACE_EXISTING => 'Replace existing nodes',
       FEEDS_UPDATE_EXISTING => 'Update existing nodes (slower than replacing them)',
+      FEEDS_SKIP_NEW => 'Update existing nodes only (Do not create new nodes)',
     );
     return $form;
   }
Only in feeds.a/plugins: FeedsNodeProcessor.inc.orig
diff -rup feeds.b/plugins/FeedsProcessor.inc feeds.a/plugins/FeedsProcessor.inc
--- feeds.b/plugins/FeedsProcessor.inc	2012-10-11 21:46:10.000000000 -0400
+++ feeds.a/plugins/FeedsProcessor.inc	2012-10-31 00:10:07.684386000 -0400
@@ -9,6 +9,7 @@
 define('FEEDS_SKIP_EXISTING', 0);
 define('FEEDS_REPLACE_EXISTING', 1);
 define('FEEDS_UPDATE_EXISTING', 2);
+define('FEEDS_SKIP_NEW',3);
 
 // Default limit for creating items on a page load, not respected by all
 // processors.
@@ -142,6 +143,12 @@ abstract class FeedsProcessor extends Fe
         continue;
       }
 
+      // Do not proceed if the item does not exist and we are set for update only
+      $skip_create = $this->config('update_existing') == FEEDS_SKIP_NEW;
+      if (empty($entity_id) && $skip_create) {
+	continue;
+      }
+
       try {
 
         // Build a new entity.
Only in feeds.a: unique_field.patch