diff --git a/nanobot/webui/settings_api.py b/nanobot/webui/settings_api.py
index e6831e3bc..315c7e2f3 100644
--- a/nanobot/webui/settings_api.py
+++ b/nanobot/webui/settings_api.py
@@ -269,7 +269,11 @@ def update_model_call_order(
     config_path: Path | None = None,
 ) -> dict[str, Any]:
     config = _load_settings_config(config_path)
-    if models.update_model_call_order(config, query):
+    if models.update_model_call_order(
+        config,
+        query,
+        oauth_status=_oauth_provider_status,
+    ):
         _save_settings_config(config, config_path)
     return settings_payload(config_path=config_path)
 
diff --git a/nanobot/webui/settings_models.py b/nanobot/webui/settings_models.py
index a6ef7b624..d819bb632 100644
--- a/nanobot/webui/settings_models.py
+++ b/nanobot/webui/settings_models.py
@@ -1312,7 +1312,12 @@ def update_model_configuration(
     return changed
 
 
-def update_model_call_order(config: Config, query: QueryParams) -> bool:
+def update_model_call_order(
+    config: Config,
+    query: QueryParams,
+    *,
+    oauth_status: OAuthStatusReader,
+) -> bool:
     raw_order = query_first_alias(query, "order", "presetNames")
     if raw_order is None:
         raise WebUISettingsError("model call order is required")
@@ -1331,15 +1336,16 @@ def update_model_call_order(config: Config, query: QueryParams) -> bool:
         raise WebUISettingsError("model call order must contain at least one preset")
 
     normalized_order = [cast(str, name).strip() for name in cast(list[object], order)]
+    unknown = [name for name in normalized_order if name not in config.model_presets]
+    if unknown:
+        raise WebUISettingsError(f"unknown model preset: {unknown[0]}")
+
     _, editable = _model_call_order_state(config)
-    if not editable:
+    if not editable and _legacy_model_configuration_migratable(config, oauth_status):
         raise WebUISettingsError(
             "convert the existing model configuration to presets first",
             status=409,
         )
-    unknown = [name for name in normalized_order if name not in config.model_presets]
-    if unknown:
-        raise WebUISettingsError(f"unknown model preset: {unknown[0]}")
 
     defaults = config.agents.defaults
     fallback_models: list[FallbackCandidate] = list(normalized_order[1:])
diff --git a/tests/webui/test_settings_api.py b/tests/webui/test_settings_api.py
index 790f9016e..a6b8c29e6 100644
--- a/tests/webui/test_settings_api.py
+++ b/tests/webui/test_settings_api.py
@@ -592,7 +592,7 @@ def test_update_model_call_order_sets_primary_and_fallbacks(
     assert saved.agents.defaults.fallback_models == ["primary"]
 
 
-def test_update_model_call_order_requires_named_primary(
+def test_update_model_call_order_activates_existing_named_preset(
     tmp_path,
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
@@ -602,11 +602,36 @@ def test_update_model_call_order_requires_named_primary(
     save_config(config, config_path)
     monkeypatch.setattr("nanobot.config.loader._current_config_path", config_path)
 
+    payload = update_model_call_order({"order": [json.dumps(["backup"])]})
+
+    assert payload["model_call_order"] == ["backup"]
+    assert payload["model_call_order_editable"] is True
+    assert load_config(config_path).agents.defaults.model_preset == "backup"
+
+
+def test_update_model_call_order_preserves_real_legacy_configuration(
+    tmp_path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    config_path = tmp_path / "config.json"
+    config = Config()
+    config.providers.openai.api_key = "sk-test"
+    config.agents.defaults.model = "openai/gpt-4o"
+    config.agents.defaults.provider = "openai"
+    config.model_presets["backup"] = ModelPresetConfig(
+        model="openai/gpt-4.1-mini",
+        provider="openai",
+    )
+    save_config(config, config_path)
+    monkeypatch.setattr("nanobot.config.loader._current_config_path", config_path)
+
     with pytest.raises(WebUISettingsError) as error:
         update_model_call_order({"order": [json.dumps(["backup"])]})
 
     assert error.value.status == 409
-    assert load_config(config_path).agents.defaults.model_preset is None
+    saved = load_config(config_path)
+    assert saved.agents.defaults.model_preset is None
+    assert saved.agents.defaults.model == "openai/gpt-4o"
 
 
 def test_migrate_model_configurations_preserves_legacy_chain(