diff --git a/netbox_custom_objects/models.py b/netbox_custom_objects/models.py
index 9ec2d4c..11e1711 100644
--- a/netbox_custom_objects/models.py
+++ b/netbox_custom_objects/models.py
@@ -12,7 +12,7 @@
 # from django.contrib.contenttypes.management import create_contenttypes
 from django.contrib.contenttypes.models import ContentType
 from django.core.validators import RegexValidator, ValidationError
-from django.db import connection, models
+from django.db import connection, models, IntegrityError
 from django.db.models import Q
 from django.db.models.functions import Lower
 from django.db.models.signals import pre_delete
@@ -52,6 +52,13 @@
 from netbox_custom_objects.constants import APP_LABEL
 from netbox_custom_objects.field_types import FIELD_TYPE_CLASS
 
+
+class UniquenessConstraintTestError(Exception):
+    """Custom exception used to signal successful uniqueness constraint test."""
+
+    pass
+
+
 USER_TABLE_DATABASE_NAME_PREFIX = "custom_objects_"
 
 
@@ -310,7 +317,10 @@ def _fetch_and_generate_field_attrs(
         for field in fields:
             field_type = FIELD_TYPE_CLASS[field.type]()
             if skip_object_fields:
-                if field.type in [CustomFieldTypeChoices.TYPE_OBJECT, CustomFieldTypeChoices.TYPE_MULTIOBJECT]:
+                if field.type in [
+                    CustomFieldTypeChoices.TYPE_OBJECT,
+                    CustomFieldTypeChoices.TYPE_MULTIOBJECT,
+                ]:
                     continue
 
             field_name = field.name
@@ -453,7 +463,9 @@ def get_model(
             "custom_object_type_id": self.id,
         }
 
-        field_attrs = self._fetch_and_generate_field_attrs(fields, skip_object_fields=skip_object_fields)
+        field_attrs = self._fetch_and_generate_field_attrs(
+            fields, skip_object_fields=skip_object_fields
+        )
 
         attrs.update(**field_attrs)
 
@@ -587,7 +599,7 @@ class CustomObjectTypeField(CloningMixin, ExportTemplatesMixin, ChangeLoggedMode
     name = models.CharField(
         verbose_name=_("name"),
         max_length=50,
-        help_text=_("Internal field name, e.g. \"vendor_label\""),
+        help_text=_('Internal field name, e.g. "vendor_label"'),
         validators=(
             RegexValidator(
                 regex=r"^[a-z0-9_]+$",
@@ -616,7 +628,9 @@ class CustomObjectTypeField(CloningMixin, ExportTemplatesMixin, ChangeLoggedMode
         verbose_name=_("group name"),
         max_length=50,
         blank=True,
-        help_text=_("Custom object fields within the same group will be displayed together"),
+        help_text=_(
+            "Custom object fields within the same group will be displayed together"
+        ),
     )
     description = models.CharField(
         verbose_name=_("description"), max_length=200, blank=True
@@ -862,6 +876,40 @@ def clean(self):
                 {"unique": _("Uniqueness cannot be enforced for boolean fields")}
             )
 
+        # Check if uniqueness constraint can be applied when changing from non-unique to unique
+        if (
+            self.pk
+            and self.unique
+            and not self.original.unique
+            and not self._state.adding
+        ):
+            field_type = FIELD_TYPE_CLASS[self.type]()
+            model_field = field_type.get_model_field(self)
+            model = self.custom_object_type.get_model()
+            model_field.contribute_to_class(model, self.name)
+
+            old_field = field_type.get_model_field(self.original)
+            old_field.contribute_to_class(model, self._original_name)
+
+            try:
+                with connection.schema_editor() as test_schema_editor:
+                    test_schema_editor.alter_field(model, old_field, model_field)
+                    # If we get here, the constraint was applied successfully
+                    # Now raise a custom exception to rollback the test transaction
+                    raise UniquenessConstraintTestError()
+            except UniquenessConstraintTestError:
+                # The constraint can be applied, validation passes
+                pass
+            except IntegrityError:
+                # The constraint cannot be applied due to existing non-unique values
+                raise ValidationError(
+                    {
+                        "unique": _(
+                            "Custom objects with non-unique values already exist so this action isn't permitted"
+                        )
+                    }
+                )
+
         # Choice set must be set on selection fields, and *only* on selection fields
         if self.type in (
             CustomFieldTypeChoices.TYPE_SELECT,