Don't crash when an altered column's old class is a custom Column subclass

piccolo/apps/migrations/auto/schema_differ.py

@@ -516,6 +516,7 @@ def alter_columns(self) -> AlterStatements:
                             expect_conflict_with_global_name=getattr(
                                 UniqueGlobalNames,
                                 f"COLUMN_{alter_column.old_column_class.__name__.upper()}",  # noqa: E501
+                                None,
                             ),
                         )
                     )

tests/apps/migrations/auto/test_schema_differ.py

@@ -450,6 +450,53 @@ def test_alter_column_precision(self) -> None:
             "manager.alter_column(table_class_name='Ticket', tablename='ticket', column_name='price', db_column_name='price', params={'digits': (4, 2)}, old_params={'digits': (5, 2)}, column_class=Numeric, old_column_class=Numeric, schema=None)",  # noqa
         )
 
+    def test_alter_column_with_custom_old_column_class(self) -> None:
+        """
+        A column whose *old* class is a user-defined ``Column`` subclass must
+        not crash the differ.
+
+        ``UniqueGlobalNames`` only knows the built-in column classes, so the
+        lookup for the old class has to tolerate a miss the same way the lookup
+        for the new class already does.
+
+        https://github.com/piccolo-orm/piccolo/issues/1428
+
+        """
+
+        class CustomVarchar(Varchar):
+            pass
+
+        name_1 = Varchar(length=20)
+        name_1._meta.name = "name"
+
+        name_2 = CustomVarchar(length=10)
+        name_2._meta.name = "name"
+
+        schema: list[DiffableTable] = [
+            DiffableTable(
+                class_name="Band",
+                tablename="band",
+                columns=[name_1],
+            )
+        ]
+        schema_snapshot: list[DiffableTable] = [
+            DiffableTable(
+                class_name="Band",
+                tablename="band",
+                columns=[name_2],
+            )
+        ]
+
+        schema_differ = SchemaDiffer(
+            schema=schema, schema_snapshot=schema_snapshot, auto_input="y"
+        )
+
+        self.assertEqual(len(schema_differ.alter_columns.statements), 1)
+        self.assertIn(
+            "old_column_class=CustomVarchar",
+            schema_differ.alter_columns.statements[0],
+        )
+
     def test_db_column_name(self) -> None:
         """
         Make sure alter statements use the ``db_column_name`` if provided.