TST: to_replace changing dtype of series (#50664)

Author: pooja-subramaniam

pandas/tests/series/methods/test_replace.py

@@ -675,3 +675,18 @@ def test_replace_value_none_dtype_numeric(self, val):
         result = ser.replace(val, None)
         expected = pd.Series([1, None], dtype=object)
         tm.assert_series_equal(result, expected)
+
+    def test_replace_change_dtype_series(self):
+        # GH#25797
+        df = pd.DataFrame.from_dict({"Test": ["0.5", True, "0.6"]})
+        df["Test"] = df["Test"].replace([True], [np.nan])
+        expected = pd.DataFrame.from_dict({"Test": ["0.5", np.nan, "0.6"]})
+        tm.assert_frame_equal(df, expected)
+
+        df = pd.DataFrame.from_dict({"Test": ["0.5", None, "0.6"]})
+        df["Test"] = df["Test"].replace([None], [np.nan])
+        tm.assert_frame_equal(df, expected)
+
+        df = pd.DataFrame.from_dict({"Test": ["0.5", None, "0.6"]})
+        df["Test"] = df["Test"].fillna(np.nan)
+        tm.assert_frame_equal(df, expected)