diff --git a/phpBB/includes/db/migration/migration.php b/phpBB/includes/db/migration/migration.php
index 4271751362..cf1e97b3b1 100644
--- a/phpBB/includes/db/migration/migration.php
+++ b/phpBB/includes/db/migration/migration.php
@@ -83,6 +83,21 @@ abstract class phpbb_db_migration
 		return array();
 	}
 
+	/**
+	* Allows you to check if the migration is effectively installed (entirely optionall)
+	*
+	* This is checked when a migration is installed. If true is returned, the migration will be set as
+	* installed without performing the database changes.
+	* This function is intended to help moving to migrations from a previous database updater, where some
+	* migrations may have been installed already even though they are not yet listed in the migrations table.
+	*
+	* @return bool True if this migration is installed, False if this migration is not installed (checked on install)
+	*/
+	public function effectively_installed()
+	{
+		return false;
+	}
+
 	/**
 	* Updates the database schema by providing a set of change instructions
 	*
diff --git a/phpBB/includes/db/migrator.php b/phpBB/includes/db/migrator.php
index 6b249e3ee0..b56da95b1a 100644
--- a/phpBB/includes/db/migrator.php
+++ b/phpBB/includes/db/migrator.php
@@ -271,10 +271,24 @@ class phpbb_db_migrator
 			'class'	=> $migration,
 		);
 
-		if (!isset($this->migration_state[$name]))
+		if ($migration->effectively_installed())
 		{
-			$state['migration_start_time'] = time();
-			$this->insert_migration($name, $state);
+			$state = array(
+				'migration_depends_on'	=> $migration->depends_on(),
+				'migration_schema_done' => true,
+				'migration_data_done'	=> true,
+				'migration_data_state'	=> '',
+				'migration_start_time'	=> 0,
+				'migration_end_time'	=> 0,
+			);
+		}
+		else
+		{
+			if (!isset($this->migration_state[$name]))
+			{
+				$state['migration_start_time'] = time();
+				$this->insert_migration($name, $state);
+			}
 		}
 
 		if (!$state['migration_schema_done'])
diff --git a/tests/dbal/migration/installed.php b/tests/dbal/migration/installed.php
new file mode 100644
index 0000000000..01829f7a99
--- /dev/null
+++ b/tests/dbal/migration/installed.php
@@ -0,0 +1,30 @@
+<?php
+/**
+*
+* @package testing
+* @copyright (c) 2011 phpBB Group
+* @license http://opensource.org/licenses/gpl-license.php GNU Public License
+*
+*/
+
+class phpbb_dbal_migration_installed extends phpbb_db_migration
+{
+	function effectively_installed()
+	{
+		return true;
+	}
+
+	function update_data()
+	{
+		return array(
+			array('custom', array(array(&$this, 'test'))),
+		);
+	}
+
+	function test()
+	{
+		global $migrator_test_installed_failed;
+
+		$migrator_test_installed_failed = true;
+	}
+}
diff --git a/tests/dbal/migrator_test.php b/tests/dbal/migrator_test.php
index 69db7ca047..9460e76f37 100644
--- a/tests/dbal/migrator_test.php
+++ b/tests/dbal/migrator_test.php
@@ -19,6 +19,7 @@ require_once dirname(__FILE__) . '/migration/recall.php';
 require_once dirname(__FILE__) . '/migration/revert.php';
 require_once dirname(__FILE__) . '/migration/revert_with_dependency.php';
 require_once dirname(__FILE__) . '/migration/fail.php';
+require_once dirname(__FILE__) . '/migration/installed.php';
 
 class phpbb_dbal_migrator_test extends phpbb_database_test_case
 {
@@ -233,4 +234,24 @@ class phpbb_dbal_migrator_test extends phpbb_database_test_case
 			$this->fail('Revert did not remove test_column.');
 		}
 	}
+
+	public function test_installed()
+	{
+		$this->migrator->set_migrations(array('phpbb_dbal_migration_installed'));
+
+		global $migrator_test_installed_failed;
+		$migrator_test_installed_failed = false;
+
+		while (!$this->migrator->finished())
+		{
+			$this->migrator->update();
+		}
+
+		$this->assertTrue($this->migrator->migration_state('phpbb_dbal_migration_installed') !== false);
+
+		if ($migrator_test_installed_failed)
+		{
+			$this->fail('Installed test failed');
+		}
+	}
 }