diff --git a/phpcs.xml.dist b/phpcs.xml.dist
index c742021..c1cb878 100644
--- a/phpcs.xml.dist
+++ b/phpcs.xml.dist
@@ -5,6 +5,7 @@
     <file>./src</file>
     <file>./tests</file>
     <exclude-pattern>*/Fixtures/*</exclude-pattern>
+    <exclude-pattern>*/LegacyClassnames.php</exclude-pattern>
 
     <!-- Excludes for PHPCS Code Style -->
     <rule ref="PSR1.Classes.ClassDeclaration.MissingNamespace">
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index a507cf9..f186f18 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -21,7 +21,11 @@
 
     <filter>
         <whitelist>
-            <directory>./src/</directory>
+            <directory>./src</directory>
+            <exclude>
+                <directory>./src/Standards/Typo3Update/Sniffs/</directory>
+                <file>./src/Standards/Typo3Update/Configuration/LegacyClassnames.php</file>
+            </exclude>
         </whitelist>
     </filter>
 </phpunit>
diff --git a/src/Standards/Typo3Update/Feature/LegacyClassnameFeature.php b/src/Standards/Typo3Update/Feature/LegacyClassnameFeature.php
index 0a25222..4b2d50c 100644
--- a/src/Standards/Typo3Update/Feature/LegacyClassnameFeature.php
+++ b/src/Standards/Typo3Update/Feature/LegacyClassnameFeature.php
@@ -100,7 +100,7 @@ class LegacyClassnameFeature implements FeatureInterface
             return false;
         }
 
-        return $this->legacyMapping->isLegacyClassname($classname);
+        return $this->legacyMapping->isCaseInsensitiveLegacyClassname($classname);
     }
 
     /**
diff --git a/src/Standards/Typo3Update/Feature/LegacyClassnameMapping.php b/src/Standards/Typo3Update/Feature/LegacyClassnameMapping.php
index a60c92b..47201c5 100644
--- a/src/Standards/Typo3Update/Feature/LegacyClassnameMapping.php
+++ b/src/Standards/Typo3Update/Feature/LegacyClassnameMapping.php
@@ -45,6 +45,7 @@ final class LegacyClassnameMapping
 
         return static::$instance;
     }
+    // @codeCoverageIgnoreStart
     private function __clone()
     {
     }
@@ -54,6 +55,7 @@ final class LegacyClassnameMapping
     private function __wakeup()
     {
     }
+    // @codeCoverageIgnoreEnd
     private function __construct()
     {
         if (is_file(Options::getMappingFile())) {
@@ -109,12 +111,21 @@ final class LegacyClassnameMapping
      * @param string $classname
      * @return bool
      */
-    public function isLegacyClassname($classname, $insensitive = true)
+    public function isLegacyClassname($classname)
     {
-        $lowerVersion = $classname;
-        if ($insensitive) {
-            $classname = strtolower($classname);
-        }
+        return $this->isLegacyTypo3Classname($classname) || $this->isLegacyMappingClassname($classname);
+    }
+
+    /**
+     * Checks whether a mapping exists for the given $classname,
+     * indicating it's legacy.
+     *
+     * @param string $classname
+     * @return bool
+     */
+    public function isCaseInsensitiveLegacyClassname($classname)
+    {
+        $lowerVersion = strtolower($classname);
 
         return $this->isLegacyTypo3Classname($classname) || $this->isLegacyMappingClassname($classname)
             || $this->isLegacyTypo3Classname($lowerVersion) || $this->isLegacyMappingClassname($lowerVersion);
@@ -130,7 +141,7 @@ final class LegacyClassnameMapping
             return $this->typo3Mappings[$this->getTypo3MappingKey($classname)];
         }
 
-        return $this>mappings[$this->getLegacyMappingKey($classname)];
+        return $this->mappings[$this->getLegacyMappingKey($classname)];
     }
 
     /**
@@ -139,12 +150,7 @@ final class LegacyClassnameMapping
      */
     protected function getTypo3MappingKey($classname)
     {
-        $lowerVersion = strtolower($classname);
-        if (isset($this->typo3MappingsKeys[$lowerVersion])) {
-            return $this->typo3MappingsKeys[$lowerVersion];
-        }
-
-        return $classname;
+        return $this->typo3MappingsKeys[strtolower($classname)];
     }
 
     /**
@@ -153,12 +159,7 @@ final class LegacyClassnameMapping
      */
     protected function getLegacyMappingKey($classname)
     {
-        $lowerVersion = strtolower($classname);
-        if (isset($this->mappingsKeys[$lowerVersion])) {
-            return $this->mappingsKeys[$lowerVersion];
-        }
-
-        return $classname;
+        return $this->mappingsKeys[strtolower($classname)];
     }
 
     /**
@@ -167,7 +168,7 @@ final class LegacyClassnameMapping
      */
     protected function isLegacyTypo3Classname($classname)
     {
-        return isset($this->typo3MappingsKeys[$classname]);
+        return isset($this->typo3Mappings[$classname]) || isset($this->typo3MappingsKeys[$classname]);
     }
 
     /**
@@ -176,7 +177,7 @@ final class LegacyClassnameMapping
      */
     protected function isLegacyMappingClassname($classname)
     {
-        return isset($this->mappingsKeys[$classname]);
+        return isset($this->mappings[$classname]) || isset($this->mappingsKeys[$classname]);
     }
 
     /**
@@ -210,6 +211,7 @@ final class LegacyClassnameMapping
 
     /**
      * Used to persist new mappings.
+     * @codeCoverageIgnore
      */
     public function __destruct()
     {
diff --git a/tests/Feature/LegacyClassnameMappingTest.php b/tests/Feature/LegacyClassnameMappingTest.php
index f420e7f..ea47cd4 100644
--- a/tests/Feature/LegacyClassnameMappingTest.php
+++ b/tests/Feature/LegacyClassnameMappingTest.php
@@ -64,14 +64,14 @@ class LegacyClassnameMappingTest extends TestCase
     /**
      * @test
      */
-    public function insensitivityLookupWorks()
+    public function inCaseSensitivityLookupWorks()
     {
         $this->assertFalse(
-            $this->subject->isLegacyClassname('Tx_About_Controller_Aboutcontroller', false),
+            $this->subject->isLegacyClassname('Tx_Extbase_Domain_Model_Backenduser'),
             'Classname was returned to be legacy but should not due to lowercase version and case sensitivity.'
         );
         $this->assertTrue(
-            $this->subject->isLegacyClassname('Tx_About_Controller_Aboutcontroller'),
+            $this->subject->isCaseInsensitiveLegacyClassname('Tx_Extbase_Domain_Model_Backenduser'),
             'Classname was not returned to be legacy but should due to case insensitivity.'
         );
     }
@@ -79,6 +79,41 @@ class LegacyClassnameMappingTest extends TestCase
     /**
      * @test
      */
+    public function weCanRetrieveNewClassname()
+    {
+        $this->assertSame(
+            'TYPO3\CMS\Extbase\Command\HelpCommandController',
+            $this->subject->getNewClassname('Tx_Extbase_Command_HelpCommandController'),
+            'New class name could not be fetched.'
+        );
+    }
+
+    /**
+     * @test
+     */
+    public function nothingWillBePersistedUntilWeAddSomething()
+    {
+        $this->subject->persistMappings();
+        $this->assertSame(
+            file_get_contents($this->getFixturePath('MappingContent.php')),
+            file_get_contents(vfsStream::url('root/LegacyClassnames.php')),
+            'File content should not be changed.'
+        );
+
+        $this->subject->addLegacyClassname('Tx_ExtName_Controller_ExampleController', '\\Vendor\\ExtName\\Controller\\ExampleController');
+        $this->subject->persistMappings();
+
+        $this->assertSame(
+            file_get_contents($this->getFixturePath('ExpectedMappingContent.php')),
+            file_get_contents(vfsStream::url('root/LegacyClassnames.php')),
+            'File content is not changed as expected.'
+        );
+    }
+
+    /**
+     * @test
+     * @runInSeparateProcess Because of file operations.
+     */
     public function addingLegacyClassnamesWillAdjustLookupAndBePersisted()
     {
         $this->assertFalse(
@@ -92,13 +127,19 @@ class LegacyClassnameMappingTest extends TestCase
             $this->subject->isLegacyClassname('Tx_ExtName_Controller_ExampleController'),
             'Classname is configured but not returned to be legacy.'
         );
+    }
 
-        $this->subject->persistMappings();
-
+    /**
+     * @test
+     * @runInSeparateProcess Because of file operations.
+     */
+    public function weCanRetrieveNewClassnameAddedBefore()
+    {
+        $this->subject->addLegacyClassname('Tx_ExtName_Controller_ExampleController', '\\Vendor\\ExtName\\Controller\\ExampleController');
         $this->assertSame(
-            file_get_contents($this->getFixturePath('ExpectedMappingContent.php')),
-            file_get_contents(vfsStream::url('root/LegacyClassnames.php')),
-            'Persisted mappings are not as expected.'
+            '\\Vendor\\ExtName\\Controller\\ExampleController',
+            $this->subject->getNewClassname('Tx_ExtName_Controller_ExampleController'),
+            'New class name could not be fetched.'
         );
     }