diff --git a/_config/extensions.yml b/_config/extensions.yml
new file mode 100644
index 0000000..3e243dc
--- /dev/null
+++ b/_config/extensions.yml
@@ -0,0 +1,6 @@
+---
+Name: userformsextensions
+---
+SilverStripe\UserForms\Model\UserDefinedForm:
+  extensions:
+    UpgradePolymorphicExtension: SilverStripe\UserForms\Extension\UpgradePolymorphicExtension
diff --git a/code/Extension/UpgradePolymorphicExtension.php b/code/Extension/UpgradePolymorphicExtension.php
new file mode 100644
index 0000000..6bf79d4
--- /dev/null
+++ b/code/Extension/UpgradePolymorphicExtension.php
@@ -0,0 +1,96 @@
+<?php
+
+namespace SilverStripe\UserForms\Extension;
+
+use SilverStripe\Control\Director;
+use SilverStripe\Core\Injector\Injector;
+use SilverStripe\ORM\DataExtension;
+use SilverStripe\ORM\DataList;
+use SilverStripe\ORM\DataObject;
+use SilverStripe\ORM\ValidationException;
+use SilverStripe\UserForms\Model\EditableFormField;
+use SilverStripe\UserForms\Model\Recipient\EmailRecipient;
+use SilverStripe\UserForms\Model\Submission\SubmittedForm;
+use SilverStripe\UserForms\Model\UserDefinedForm;
+use SilverStripe\UserForms\UserForm;
+
+/**
+ * This extension provides a hook that runs during a dev/build which will check for existing data in various
+ * polymorphic relationship fields for userforms models, and ensure that the data is correct.
+ *
+ * Various `Parent` relationships in silverstripe/userforms for SilverStripe 3 were mapped directly to UserDefinedForm
+ * instances, and were made polymorphic in SilverStripe 4 (which also requires a class name). This means that a
+ * certain amount of manual checking is required to ensure that upgrades are performed smoothly.
+ *
+ * @internal This API is likely to be removed in later major versions of silverstripe/userforms
+ */
+class UpgradePolymorphicExtension extends DataExtension
+{
+    /**
+     * A list of userforms classes that have had polymorphic relationships added in SilverStripe 4, and the fields
+     * on them that are polymorphic
+     *
+     * @var array
+     */
+    protected $targets = [
+        EditableFormField::class => ['ParentClass'],
+        EmailRecipient::class => ['FormClass'],
+        SubmittedForm::class => ['ParentClass'],
+    ];
+
+    /**
+     * The default class name that will be used to replace values with
+     *
+     * @var string
+     */
+    protected $defaultReplacement = UserDefinedForm::class;
+
+    public function requireDefaultRecords()
+    {
+        if (!UserDefinedForm::config()->get('upgrade_on_build')) {
+            return;
+        }
+
+        $updated = 0;
+        foreach ($this->targets as $className => $fieldNames) {
+            foreach ($fieldNames as $fieldName) {
+                /** @var DataList $list */
+                $list = $className::get();
+
+                foreach ($list as $entry) {
+                    /** @var DataObject $relationshipObject */
+                    $relationshipObject = Injector::inst()->get($entry->$fieldName);
+                    if (!$relationshipObject) {
+                        continue;
+                    }
+
+                    // If the defined data class doesn't have the UserForm trait applied, it's probably wrong. Re-map
+                    // it to a default value that does
+                    $classTraits = class_uses($relationshipObject);
+                    if (in_array(UserForm::class, $classTraits)) {
+                        continue;
+                    }
+
+                    $entry->$fieldName = $this->defaultReplacement;
+                    try {
+                        $entry->write();
+                        $updated++;
+                    } catch (ValidationException $ex) {
+                        // no-op, allow the rest of dev/build to continue. There may be an error indicating that the
+                        // object's class doesn't exist, which can be fixed by {@link DatabaseAdmin::doBuild} and this
+                        // logic will work the next time dev/build is run.
+                    }
+                }
+            }
+        }
+
+        if ($updated) {
+            $message = "Corrected {$updated} default polymorphic class names to {$this->defaultReplacement}";
+            if (Director::is_cli()) {
+                echo sprintf(" * %s\n", $message);
+            } else {
+                echo sprintf("<li>%s</li>\n", $message);
+            }
+        }
+    }
+}
diff --git a/lang/en.yml b/lang/en.yml
index 1fc9461..e375dcf 100644
--- a/lang/en.yml
+++ b/lang/en.yml
@@ -3,8 +3,6 @@ en:
     SELECTUPLOADFOLDER: 'Select upload folder'
   SilverStripe\CMS\Model\SiteTree:
     TABMAIN: Main
-  SilverStripe\Forms\Form:
-    FIELDISREQUIRED: '{name} is required.'
   SilverStripe\UserForms:
     ADDEMAILRECIPIENT: 'Add Email Recipient'
     CLEARBUTTON: Clear