Backport PR #31545: BUG&TST: df.replace fail after converting to new dtype (#31584)

Co-authored-by: Kaiqi Dong <[email protected]>

Author: jreback

pandas/tests/frame/methods/test_replace.py

@@ -1356,3 +1356,10 @@ def test_replace_replacer_dtype(self, replacer):
         result = df.replace({"a": replacer, "b": replacer})
         expected = pd.DataFrame([replacer])
         tm.assert_frame_equal(result, expected)
+
+    def test_replace_after_convert_dtypes(self):
+        # GH31517
+        df = pd.DataFrame({"grp": [1, 2, 3, 4, 5]}, dtype="Int64")
+        result = df.replace(1, 10)
+        expected = pd.DataFrame({"grp": [10, 2, 3, 4, 5]}, dtype="Int64")
+        tm.assert_frame_equal(result, expected)