diff --git a/.chronus/changes/storage_casing-2024-5-5-15-48-15.md b/.chronus/changes/storage_casing-2024-5-5-15-48-15.md
new file mode 100644
index 0000000000..1a5344ae2e
--- /dev/null
+++ b/.chronus/changes/storage_casing-2024-5-5-15-48-15.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@autorest/python"
+---
+
+fix casing of multipart legacy properties
\ No newline at end of file
diff --git a/packages/autorest.python/autorest/m4reformatter/__init__.py b/packages/autorest.python/autorest/m4reformatter/__init__.py
index ab4cfa8fd8..9ae4a5cd4d 100644
--- a/packages/autorest.python/autorest/m4reformatter/__init__.py
+++ b/packages/autorest.python/autorest/m4reformatter/__init__.py
@@ -700,7 +700,7 @@ def update_flattened_parameter(
         # this means i'm a property that is part of a flattened model
         target_property_name = yaml_data["targetProperty"]["language"]["default"]["name"]
         param = self.update_parameter(yaml_data)
-        body_parameter.setdefault("propertyToParameterName", {})[target_property_name] = param["clientName"]
+        body_parameter.setdefault("propertyToParameterName", {})[target_property_name.lower()] = param["clientName"]
         param["inFlattenedBody"] = True
         return param
 
@@ -829,7 +829,7 @@ def update_parameters(
                     prop
                     for prop in grouper["type"]["properties"]
                     if p["clientName"].lstrip("_") in prop["groupedParameterNames"]  # TODO: patching m4
-                )["clientName"]: p["clientName"]
+                )["clientName"].lower(): p["clientName"]
                 for p in all_params
                 if p.get("groupedBy") == grouper_name
             }