BUG: Series(category).replace to maintain order of categories (#51057)

* BUG: Series(category).replace to maintain order of categories

* fix

Author: lukemanley

pandas/core/arrays/categorical.py

@@ -2284,7 +2284,16 @@ def _replace(self, *, to_replace, value, inplace: bool = False):
         ser = ser.replace(to_replace=to_replace, value=value)
 
         all_values = Index(ser)
-        new_categories = Index(ser.drop_duplicates(keep="first"))
+
+        # GH51016: maintain order of existing categories
+        idxr = cat.categories.get_indexer_for(all_values)
+        locs = np.arange(len(ser))
+        locs = np.where(idxr == -1, locs, idxr)
+        locs = locs.argsort()
+
+        new_categories = ser.take(locs)
+        new_categories = new_categories.drop_duplicates(keep="first")
+        new_categories = Index(new_categories)
         new_codes = recode_for_categories(
             cat._codes, all_values, new_categories, copy=False
         )

pandas/tests/arrays/categorical/test_replace.py

@@ -78,3 +78,13 @@ def test_replace_categorical_ea_dtype():
     result = pd.Series(cat).replace(["a", "b"], ["c", pd.NA])._values
     expected = Categorical(pd.array(["c", pd.NA], dtype="string"))
     tm.assert_categorical_equal(result, expected)
+
+
+def test_replace_maintain_ordering():
+    # GH51016
+    dtype = pd.CategoricalDtype([0, 1, 2], ordered=True)
+    ser = pd.Series([0, 1, 2], dtype=dtype)
+    result = ser.replace(0, 2)
+    expected_dtype = pd.CategoricalDtype([1, 2], ordered=True)
+    expected = pd.Series([2, 1, 2], dtype=expected_dtype)
+    tm.assert_series_equal(expected, result, check_category_order=True)