DOC: add whatsnew

Author: arw2019

doc/source/whatsnew/v1.2.0.rst

@@ -528,6 +528,7 @@ ExtensionArray
 - Fixed bug where ``astype()`` with equal dtype and ``copy=False`` would return a new object (:issue:`284881`)
 - Fixed bug when applying a NumPy ufunc with multiple outputs to a :class:`pandas.arrays.IntegerArray` returning None (:issue:`36913`)
 - Fixed an inconsistency in :class:`PeriodArray`'s ``__init__`` signature to those of :class:`DatetimeArray` and :class:`TimedeltaArray` (:issue:`37289`)
+- Bug in :meth:`Series.replace` did not preserve ``dtype`` of original :class:`Series` (:issue:`33484`)
 
 Other
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