diff --git modules/filter/filter.admin.inc modules/filter/filter.admin.inc
index a1e79bc..4dd9c35 100644
--- modules/filter/filter.admin.inc
+++ modules/filter/filter.admin.inc
@@ -136,7 +136,7 @@ function filter_admin_format_form($form, &$form_state, $format) {
     '#default_value' => $format->format,
     '#maxlength' => 255,
     '#machine_name' => array(
-      'exists' => 'filter_format_load',
+      'exists' => 'filter_format_exists',
     ),
     '#disabled' => !empty($format->format),
   );
@@ -288,13 +288,13 @@ function theme_filter_admin_format_filter_order($variables) {
  * Validate text format form submissions.
  */
 function filter_admin_format_form_validate($form, &$form_state) {
-  if (!isset($form_state['values']['format'])) {
-    $format_name = trim($form_state['values']['name']);
-    form_set_value($form['name'], $format_name, $form_state);
-    $result = db_query("SELECT format FROM {filter_format} WHERE name = :name", array(':name' => $format_name))->fetchField();
-    if ($result) {
-      form_set_error('name', t('Text format names must be unique. A format named %name already exists.', array('%name' => $format_name)));
-    }
+  $format_format = trim($form_state['values']['format']);
+  form_set_value($form['format'], $format_format, $form_state);
+  $format_name = trim($form_state['values']['name']);
+  form_set_value($form['name'], $format_name, $form_state);
+  $result = db_query("SELECT format FROM {filter_format} WHERE name = :name AND format <> :format", array(':name' => $format_name, ':format' => $format_format))->fetchField();
+  if ($result) {
+    form_set_error('name', t('Text format names must be unique. A format named %name already exists.', array('%name' => $format_name)));
   }
 }
 
diff --git modules/filter/filter.module modules/filter/filter.module
index a4ce97b..f0bb109 100644
--- modules/filter/filter.module
+++ modules/filter/filter.module
@@ -153,7 +153,11 @@ function _filter_disable_format_access($format) {
  *   The format ID.
  *
  * @return
- *   A fully-populated text format object.
+ *   A fully-populated text format object, if the requested format exists and
+ *   is enabled. If the format does not exist, or exists in the database but
+ *   has been marked as disabled, FALSE is returned.
+ *
+ * @see filter_format_exists()
  */
 function filter_format_load($format_id) {
   $formats = filter_formats();
@@ -165,9 +169,10 @@ function filter_format_load($format_id) {
  *
  * @param $format
  *   A format object using the properties:
+ *   - 'format': A machine-readable name representing the ID of the text format
+ *     to save. If this corresponds to an existing text format, that format
+ *     will be updated; otherwise, a new format will be created.
  *   - 'name': The title of the text format.
- *   - 'format': (optional) The internal ID of the text format. If omitted, a
- *     new text format is created.
  *   - 'weight': (optional) The weight of the text format, which controls its
  *     placement in text format lists. If omitted, the weight is set to 0.
  *   - 'filters': (optional) An associative, multi-dimensional array of filters
@@ -266,9 +271,9 @@ function filter_format_save($format) {
  * Disable a text format.
  *
  * There is no core facility to re-enable a disabled format. It is not deleted
- * to keep information for contrib and to make sure the format auto increment
- * id is never reused. As there might be content using the disabled format,
- * this would lead to data corruption.
+ * to keep information for contrib and to make sure the format ID is never
+ * reused. As there might be content using the disabled format, this would lead
+ * to data corruption.
  *
  * @param $format
  *   The text format object to be disabled.
@@ -288,6 +293,23 @@ function filter_format_disable($format) {
 }
 
 /**
+ * Determines if a text format exists.
+ *
+ * @param $format_id
+ *   The ID of the text format to check.
+ *
+ * @return
+ *   TRUE if the text format exists, or FALSE if it does not. Note that for a
+ *   format which has been disabled (and therefore is no longer available for
+ *   use) but still exists in the database, TRUE will be returned.
+ *
+ * @see filter_format_load()
+ */
+function filter_format_exists($format_id) {
+  return (bool) db_query_range('SELECT 1 FROM {filter_format} WHERE format = :format', 0, 1, array(':format' => $format_id))->fetchField();
+}
+
+/**
  * Display a text format form title.
  */
 function filter_admin_format_title($format) {
diff --git modules/filter/filter.test modules/filter/filter.test
index f27ed0a..af1d670 100644
--- modules/filter/filter.test
+++ modules/filter/filter.test
@@ -207,6 +207,22 @@ class FilterAdminTestCase extends DrupalWebTestCase {
     // Verify that disabled text format no longer exists.
     $this->drupalGet('admin/config/content/formats/' . $format->format);
     $this->assertResponse(404, t('Disabled text format no longer exists.'));
+
+    // Attempt to create a format of the same machine name.
+    $edit = array(
+      'format' => $format->format,
+      'name' => 'New format',
+    );
+    $this->drupalPost('admin/config/content/formats/add', $edit, t('Save configuration'));
+    $this->assertText('The machine-readable name is already in use. It must be unique.');
+
+    // Attempt to create a format of the same name.
+    $edit = array(
+      'format' => 'new_format',
+      'name' => $format->name,
+    );
+    $this->drupalPost('admin/config/content/formats/add', $edit, t('Save configuration'));
+    $this->assertText('Text format names must be unique. A format named ' . check_plain($format->name) . ' already exists.');
   }
 
   /**
@@ -1082,7 +1098,7 @@ class FilterUnitTestCase extends DrupalUnitTestCase {
     $f = _filter_html('<p onerror="alert(0);" />', $filter);
     $this->assertNoNormalized($f, 'onerror', t('HTML filter should remove on* attributes on default.'));
 
-    $f = _filter_html('<code onerror>&nbsp;</code>', $filter);    
+    $f = _filter_html('<code onerror>&nbsp;</code>', $filter);
     $this->assertNoNormalized($f, 'onerror', t('HTML filter should remove empty on* attributes on default.'));
   }
 
diff --git modules/simpletest/simpletest.info modules/simpletest/simpletest.info
index ab80c84..3cc5ede 100644
--- modules/simpletest/simpletest.info
+++ modules/simpletest/simpletest.info
@@ -41,6 +41,7 @@ files[] = tests/update.test
 files[] = tests/xmlrpc.test
 files[] = tests/upgrade/upgrade.test
 files[] = tests/upgrade/upgrade.comment.test
+files[] = tests/upgrade/upgrade.filter.test
 files[] = tests/upgrade/upgrade.node.test
 files[] = tests/upgrade/upgrade.taxonomy.test
 files[] = tests/upgrade/upgrade.upload.test
diff --git modules/simpletest/tests/upgrade/upgrade.filter.test modules/simpletest/tests/upgrade/upgrade.filter.test
new file mode 100644
index 0000000..2642921
--- /dev/null
+++ modules/simpletest/tests/upgrade/upgrade.filter.test
@@ -0,0 +1,56 @@
+<?php
+// $Id$
+
+/**
+ * Upgrade test for filter format identifiers.
+ *
+ * Filter format identifiers changed from sequential ids to machine names.
+ * Verify that filter formats and references to filter formats in core are
+ * converted properly.
+ */
+class FilterFormatUpgradePathTestCase extends UpgradePathTestCase {
+  public static function getInfo() {
+    return array(
+      'name'  => 'Filter format upgrade path',
+      'description'  => 'Verifies that filter formats and references to filter formats are converted properly.',
+      'group' => 'Upgrade path',
+    );
+  }
+
+  public function setUp() {
+    // Path to the database dump.
+    $this->databaseDumpFiles = array(
+      drupal_get_path('module', 'simpletest') . '/tests/upgrade/drupal-6.filled.database.php',
+    );
+    parent::setUp();
+  }
+
+  /**
+   * Test a successful upgrade.
+   */
+  public function testFilterFormatUpgrade() {
+    $this->assertTrue($this->performUpgrade(), t('The upgrade was completed successfully.'));
+
+    $format = filter_format_load('1');
+    $this->assertTrue($format->format == '1', t('Filter format found.'));
+    $format->format = 'test_filter';
+    $format->name = 'Test filter';
+    filter_format_save($format);
+    $format = filter_format_load('test_filter');
+    $this->assertTrue($format->format == 'test_filter', t('Saved a filter format with machine name.'));
+
+    $account = user_load(4);
+    user_save($account, array('signature_format' => 'test_filter'));
+    $account = user_load(4);
+    $this->assertTrue($account->signature_format == 'test_filter', t('Signature format changed successfully to a filter format with machine name.'));
+
+    $delta = db_insert('block_custom')
+    ->fields(array(
+      'body' => 'Test block',
+      'info' => 'Test block',
+      'format' => 'test_filter',
+    ))
+    ->execute();
+    $this->assertTrue($delta > 0, t('Created a custom block using a filter format with machine name.'));
+  }
+}