BUG: Timestamp.replace failing to update unit

doc/source/whatsnew/v3.0.0.rst

@@ -901,6 +901,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.union` and :meth:`DatetimeIndex.intersection` when ``unit`` was non-nanosecond (:issue:`59036`)
 - Bug in :meth:`Index.union` with a ``pyarrow`` timestamp dtype incorrectly returning ``object`` dtype (:issue:`58421`)
 - Bug in :meth:`Series.dt.microsecond` producing incorrect results for pyarrow backed :class:`Series`. (:issue:`59154`)
+- Bug in :meth:`Timestamp.replace` failing to update ``unit`` attribute when replacement introduces non-zero ``nanosecond`` or ``microsecond`` (:issue:`57749`)
 - Bug in :meth:`to_datetime` not respecting dayfirst if an uncommon date string was passed. (:issue:`58859`)
 - Bug in :meth:`to_datetime` on float array with missing values throwing ``FloatingPointError`` (:issue:`58419`)
 - Bug in :meth:`to_datetime` on float32 df with year, month, day etc. columns leads to precision issues and incorrect result. (:issue:`60506`)

pandas/_libs/tslibs/timestamps.pyx

@@ -3357,6 +3357,7 @@ default 'raise'
             datetime ts_input
             tzinfo_type tzobj
             _TSObject ts
+            NPY_DATETIMEUNIT creso = self._creso
 
         # set to naive if needed
         tzobj = self.tzinfo
@@ -3396,8 +3397,12 @@ default 'raise'
             dts.sec = validate("second", second)
         if microsecond is not None:
             dts.us = validate("microsecond", microsecond)
+            if creso < NPY_DATETIMEUNIT.NPY_FR_us:
+                # GH#57749
+                creso = NPY_DATETIMEUNIT.NPY_FR_us
         if nanosecond is not None:
             dts.ps = validate("nanosecond", nanosecond) * 1000
+            creso = NPY_FR_ns  # GH#57749
         if tzinfo is not object:
             tzobj = tzinfo
 
@@ -3407,17 +3412,17 @@ default 'raise'
             #  to datetimes outside of pydatetime range.
             ts = _TSObject()
             try:
-                ts.value = npy_datetimestruct_to_datetime(self._creso, &dts)
+                ts.value = npy_datetimestruct_to_datetime(creso, &dts)
             except OverflowError as err:
                 fmt = dts_to_iso_string(&dts)
                 raise OutOfBoundsDatetime(
                     f"Out of bounds timestamp: {fmt} with frequency '{self.unit}'"
                 ) from err
             ts.dts = dts
-            ts.creso = self._creso
+            ts.creso = creso
             ts.fold = fold
             return create_timestamp_from_ts(
-                ts.value, dts, tzobj, fold, reso=self._creso
+                ts.value, dts, tzobj, fold, reso=creso
             )
 
         elif tzobj is not None and treat_tz_as_pytz(tzobj):
@@ -3436,10 +3441,10 @@ default 'raise'
             ts_input = datetime(**kwargs)
 
         ts = convert_datetime_to_tsobject(
-            ts_input, tzobj, nanos=dts.ps // 1000, reso=self._creso
+            ts_input, tzobj, nanos=dts.ps // 1000, reso=creso
         )
         return create_timestamp_from_ts(
-            ts.value, dts, tzobj, fold, reso=self._creso
+            ts.value, dts, tzobj, fold, reso=creso
         )
 
     def to_julian_date(self) -> np.float64:

pandas/tests/reshape/test_pivot.py

@@ -642,7 +642,7 @@ def test_pivot_tz_in_values(self):
         )
 
         df = df.set_index("ts").reset_index()
-        mins = df.ts.map(lambda x: x.replace(hour=0, minute=0, second=0, microsecond=0))
+        mins = df.ts.map(lambda x: x.replace(hour=0, minute=0, second=0))
 
         result = pivot_table(
             df.set_index("ts").reset_index(),

pandas/tests/scalar/timestamp/methods/test_replace.py

@@ -195,3 +195,17 @@ def test_replace_preserves_fold(self, fold):
         ts_replaced = ts.replace(second=1)
 
         assert ts_replaced.fold == fold
+
+    def test_replace_updates_unit(self):
+        # GH#57749
+        ts = Timestamp("2023-07-15 23:08:12.134567123")
+        ts2 = Timestamp("2023-07-15 23:08:12.000000")
+        assert ts2.unit == "us"
+        result = ts2.replace(microsecond=ts.microsecond, nanosecond=ts.nanosecond)
+        assert result.unit == "ns"
+        assert result == ts
+
+        ts3 = Timestamp("2023-07-15 23:08:12").as_unit("s")
+        result = ts3.replace(microsecond=ts2.microsecond)
+        assert result.unit == "us"
+        assert result == ts2