diff --git a/tests/repositories/plan/test_plan_postgres_repository.py b/tests/repositories/plan/test_plan_postgres_repository.py
index 2d3fb7473..7198a6926 100644
--- a/tests/repositories/plan/test_plan_postgres_repository.py
+++ b/tests/repositories/plan/test_plan_postgres_repository.py
@@ -227,3 +227,35 @@ def test_reports_which_properties_already_have_a_default_plan(
 
     # Assert — only the property with a default Plan is reported
     assert have_default == {20}
+
+
+def test_a_new_default_demotes_the_prior_default_across_scenarios(
+    db_engine: Engine,
+) -> None:
+    """One default Plan per property: a promoted first Plan (non-default
+    scenario) must lose the default when the default scenario's Plan lands —
+    readers filter portfolio + is_default only, never scenario (#1490)."""
+    # Arrange — the property's first Plan, promoted under non-default scenario 7
+    with Session(db_engine) as session:
+        first_id = PlanPostgresRepository(session).save(
+            _plan(), property_id=30, scenario_id=7, portfolio_id=1, is_default=True
+        )
+        session.commit()
+
+    # Act — the default scenario (8) models the same property
+    with Session(db_engine) as session:
+        second_id = PlanPostgresRepository(session).save(
+            _plan(), property_id=30, scenario_id=8, portfolio_id=1, is_default=True
+        )
+        session.commit()
+
+    # Assert — exactly one default for the property, the newest plan
+    with Session(db_engine) as session:
+        plan_rows = session.exec(
+            select(PlanModel).where(col(PlanModel.property_id) == 30)
+        ).all()
+        by_id = {p.id: p for p in plan_rows}
+
+    assert by_id[first_id].is_default is False  # demoted across scenarios
+    assert by_id[second_id].is_default is True
+    assert sum(1 for p in plan_rows if p.is_default) == 1