diff --git a/modules/system/classes/PluginManager.php b/modules/system/classes/PluginManager.php
index f2ba9cdbbc..e9e98e2ca8 100644
--- a/modules/system/classes/PluginManager.php
+++ b/modules/system/classes/PluginManager.php
@@ -455,8 +455,14 @@ public function findByIdentifier(PluginBase|string $identifier, bool $ignoreRepl
             return $identifier;
         }
 
-        if (!$ignoreReplacements && is_string($identifier) && isset($this->replacementMap[$identifier])) {
-            $identifier = $this->replacementMap[$identifier];
+        // TODO: we should refactor the plugin manager to handle identifiers internally as lowers
+        $replacementMap = array_combine(
+            array_map('strtolower', array_keys($this->replacementMap)),
+            array_values($this->replacementMap)
+        );
+
+        if (!$ignoreReplacements && is_string($identifier) && isset($replacementMap[strtolower($identifier)])) {
+            $identifier = $replacementMap[strtolower($identifier)];
         }
 
         if (!isset($this->plugins[$identifier])) {
@@ -473,7 +479,10 @@ public function hasPlugin(PluginBase|string $plugin): bool
     {
         $normalized = $this->getNormalizedIdentifier($plugin);
 
-        return isset($this->plugins[$normalized]) || isset($this->replacementMap[$normalized]);
+        return isset($this->plugins[$normalized])
+            || isset($this->replacementMap[$normalized])
+            // TODO: we should refactor the plugin manager to handle identifiers internally as lowers
+            || in_array(strtolower($normalized), array_map('strtolower', array_keys($this->replacementMap)));
     }
 
     /**
@@ -571,8 +580,7 @@ public function getNamespace(PluginBase|string $plugin): string
      */
     public function normalizeIdentifier(string $code): string
     {
-        $code = strtolower($code);
-        return $this->normalizedMap[$code] ?? $code;
+        return $this->normalizedMap[strtolower($code)] ?? $code;
     }
 
     /**
diff --git a/tests/fixtures/plugins/winter/replacenotinstalled/Plugin.php b/tests/fixtures/plugins/winter/replacenotinstalled/Plugin.php
new file mode 100644
index 0000000000..531018adf1
--- /dev/null
+++ b/tests/fixtures/plugins/winter/replacenotinstalled/Plugin.php
@@ -0,0 +1,18 @@
+<?php namespace Winter\ReplaceNotInstalled;
+
+use System\Classes\PluginBase;
+
+class Plugin extends PluginBase
+{
+    public function pluginDetails()
+    {
+        return [
+            'name' => 'Winter Sample Plugin',
+            'description' => 'Sample plugin used by unit tests.',
+            'author' => 'Alexey Bobkov, Samuel Georges',
+            'replaces' => [
+                'Winter.NotInstalled' => '>=1.0.3'
+            ]
+        ];
+    }
+}
diff --git a/tests/fixtures/plugins/winter/replacenotinstalled/updates/version.yaml b/tests/fixtures/plugins/winter/replacenotinstalled/updates/version.yaml
new file mode 100644
index 0000000000..979448dee1
--- /dev/null
+++ b/tests/fixtures/plugins/winter/replacenotinstalled/updates/version.yaml
@@ -0,0 +1,2 @@
+1.0.0: Initial build
+1.0.1: Updated plugin
diff --git a/tests/unit/system/classes/PluginManagerTest.php b/tests/unit/system/classes/PluginManagerTest.php
index 10667f5d29..8fb863d7b5 100644
--- a/tests/unit/system/classes/PluginManagerTest.php
+++ b/tests/unit/system/classes/PluginManagerTest.php
@@ -7,6 +7,10 @@
 
 class PluginManagerTest extends TestCase
 {
+    const INSTALLED_PLUGIN_COUNT = 13;
+    const ENABLED_PLUGIN_COUNT = 10;
+    const PLUGIN_NAMESPACE_COUNT = 14;
+
     public $manager;
     protected $output;
 
@@ -121,7 +125,7 @@ public function testLoadPlugins()
     {
         $result = $this->manager->loadPlugins();
 
-        $this->assertCount(12, $result);
+        $this->assertCount(static::INSTALLED_PLUGIN_COUNT, $result);
         $this->assertArrayHasKey('Winter.NoUpdates', $result);
         $this->assertArrayHasKey('Winter.Sample', $result);
         $this->assertArrayHasKey('Winter.Tester', $result);
@@ -163,7 +167,7 @@ public function testGetPlugins()
     {
         $result = $this->manager->getPlugins();
 
-        $this->assertCount(9, $result);
+        $this->assertCount(static::ENABLED_PLUGIN_COUNT, $result);
         $this->assertArrayHasKey('Winter.NoUpdates', $result);
         $this->assertArrayHasKey('Winter.Sample', $result);
         $this->assertArrayHasKey('Winter.Tester', $result);
@@ -223,7 +227,7 @@ public function testGetPluginNamespaces()
     {
         $result = $this->manager->getPluginNamespaces();
 
-        $this->assertCount(13, $result);
+        $this->assertCount(static::PLUGIN_NAMESPACE_COUNT, $result);
         $this->assertArrayHasKey('\winter\noupdates', $result);
         $this->assertArrayHasKey('\winter\sample', $result);
         $this->assertArrayHasKey('\winter\tester', $result);
@@ -277,7 +281,7 @@ public function testPluginDetails()
     public function testUnregisterall()
     {
         $result = $this->manager->getPlugins();
-        $this->assertCount(9, $result);
+        $this->assertCount(static::ENABLED_PLUGIN_COUNT, $result);
 
         $this->manager->unregisterAll();
         $this->assertEmpty($this->manager->getPlugins());
@@ -346,6 +350,102 @@ public function testReplacement()
         $this->assertEquals('Winter.Replacement', $this->manager->findByIdentifier('Winter.Original')->getPluginIdentifier());
     }
 
+    public function testHasPluginReplacement()
+    {
+        // check a replaced plugin
+        $this->assertTrue($this->manager->hasPlugin('Winter.Original'));
+        $this->assertTrue($this->manager->isDisabled('Winter.Original'));
+        // check a replacement plugin
+        $this->assertTrue($this->manager->hasPlugin('Winter.Replacement'));
+        $this->assertFalse($this->manager->isDisabled('Winter.Replacement'));
+        // check a plugin where the replacement is invalid
+        $this->assertTrue($this->manager->hasPlugin('Winter.InvalidReplacement'));
+        $this->assertTrue($this->manager->isDisabled('Winter.InvalidReplacement'));
+        // check a plugin replacing a plugin not found on disk
+        $this->assertTrue($this->manager->hasPlugin('Winter.ReplaceNotInstalled'));
+        $this->assertFalse($this->manager->isDisabled('Winter.ReplaceNotInstalled'));
+        // ensure searching for the alias of a replacement (plugin not installed)
+        $this->assertTrue($this->manager->hasPlugin('Winter.NotInstalled'));
+
+        $this->assertInstanceOf(\Winter\Replacement\Plugin::class, $this->manager->findByIdentifier('Winter.Original'));
+        $this->assertInstanceOf(\Winter\Replacement\Plugin::class, $this->manager->findByIdentifier('Winter.Replacement'));
+
+        // check getting a plugin via it's not installed original plugin identifier
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('Winter.NotInstalled'));
+        $this->assertNull($this->manager->findByIdentifier('Winter.NotInstalled', true));
+
+        // force getting the original plugin
+        $this->assertInstanceOf(\Winter\Original\Plugin::class, $this->manager->findByIdentifier('Winter.Original', true));
+    }
+
+    public function testHasPluginReplacementMixedCase()
+    {
+        // test checking casing of installed plugin (resolved via getNormalizedIdentifier())
+        $this->assertTrue($this->manager->hasPlugin('Winter.ReplaceNotInstalled'));
+        $this->assertTrue($this->manager->hasPlugin('Winter.replaceNotInstalled'));
+        $this->assertTrue($this->manager->hasPlugin('Winter.replacenotInstalled'));
+        $this->assertTrue($this->manager->hasPlugin('winter.replacenotInstalled'));
+        $this->assertTrue($this->manager->hasPlugin('winter.replacenotinstalled'));
+
+        // test checking casing of installed replaced plugin (resolved via getNormalizedIdentifier() & replacementMap)
+        $this->assertTrue($this->manager->hasPlugin('Winter.Original'));
+        $this->assertTrue($this->manager->hasPlugin('Winter.original'));
+        $this->assertTrue($this->manager->hasPlugin('winter.original'));
+
+        // test checking casing of uninstalled plugin (resolved via strtolower() on replacement keys)
+        $this->assertTrue($this->manager->hasPlugin('Winter.NotInstalled'));
+        $this->assertTrue($this->manager->hasPlugin('Winter.notInstalled'));
+        $this->assertTrue($this->manager->hasPlugin('winter.notInstalled'));
+        $this->assertTrue($this->manager->hasPlugin('Winter.notinstalled'));
+    }
+
+    public function testExistsReplacementMixedCase()
+    {
+        // test checking casing of installed plugin (resolved via getNormalizedIdentifier())
+        $this->assertTrue($this->manager->exists('Winter.ReplaceNotInstalled'));
+        $this->assertTrue($this->manager->exists('Winter.replaceNotInstalled'));
+        $this->assertTrue($this->manager->exists('Winter.replacenotInstalled'));
+        $this->assertTrue($this->manager->exists('winter.replacenotInstalled'));
+        $this->assertTrue($this->manager->exists('winter.replacenotinstalled'));
+
+        // test checking casing of installed replaced plugin (resolved via getNormalizedIdentifier() & replacementMap)
+        $this->assertFalse($this->manager->exists('Winter.Original'));
+        $this->assertFalse($this->manager->exists('Winter.original'));
+        $this->assertFalse($this->manager->exists('winter.original'));
+
+        // test checking casing of uninstalled plugin (resolved via strtolower() on replacement keys)
+        $this->assertTrue($this->manager->exists('Winter.NotInstalled'));
+        $this->assertTrue($this->manager->exists('Winter.notInstalled'));
+        $this->assertTrue($this->manager->exists('winter.notInstalled'));
+        $this->assertTrue($this->manager->exists('Winter.notinstalled'));
+    }
+
+    public function testFindByIdentifierReplacementMixedCase()
+    {
+        // test resolving plugin with mixed casing
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('Winter.ReplaceNotInstalled'));
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('Winter.replaceNotInstalled'));
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('Winter.replacenotInstalled'));
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('winter.replacenotInstalled'));
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('winter.replacenotinstalled'));
+
+        // test resolving replacement plugin with mixed casing
+        $this->assertInstanceOf(\Winter\Replacement\Plugin::class, $this->manager->findByIdentifier('Winter.Original'));
+        $this->assertInstanceOf(\Winter\Replacement\Plugin::class, $this->manager->findByIdentifier('Winter.original'));
+        $this->assertInstanceOf(\Winter\Replacement\Plugin::class, $this->manager->findByIdentifier('winter.original'));
+
+        // test resolving original plugin with mixed casing when ignoring replacements
+        $this->assertInstanceOf(\Winter\Original\Plugin::class, $this->manager->findByIdentifier('Winter.Original', true));
+        $this->assertInstanceOf(\Winter\Original\Plugin::class, $this->manager->findByIdentifier('Winter.original', true));
+        $this->assertInstanceOf(\Winter\Original\Plugin::class, $this->manager->findByIdentifier('winter.original', true));
+
+        // test resolving replacement plugin of uninstalled plugin with mixed casing
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('Winter.NotInstalled'));
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('Winter.notInstalled'));
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('winter.notInstalled'));
+        $this->assertInstanceOf(\Winter\ReplaceNotInstalled\Plugin::class, $this->manager->findByIdentifier('Winter.notinstalled'));
+    }
+
     public function testGetReplacements()
     {
         $replacementPluginReplaces = $this->manager->findByIdentifier('Winter.Replacement')->getReplaces();