BUG: astype('datetime64[us]') has no effect on datetime64[ns] series

[gtopper]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd

data = [
    {
        'time': pd.Timestamp('2020-01-26 12:41:37.123456789')
    },
]
df = pd.DataFrame(data, columns=['time'])

df['time'] = df['time'].astype('datetime64[us]')

print(df['time'][0])

df.to_parquet("out.pq", index=True)

Issue Description

In 1.5.0 and 1.5.1, it prints

2020-01-26 12:41:37.123456789

and raises an error:

Traceback (most recent call last):
  File "/Users/gal/work/storey/pandas-1.5.1-issue.py", line 14, in <module>
    df.to_parquet("out.pq", index=True)
  File "/Users/gal/.local/share/virtualenvs/storey-ptFrrr4H/lib/python3.8/site-packages/pandas/util/_decorators.py", line 211, in wrapper
    return func(*args, **kwargs)
  File "/Users/gal/.local/share/virtualenvs/storey-ptFrrr4H/lib/python3.8/site-packages/pandas/core/frame.py", line 2974, in to_parquet
    return to_parquet(
  File "/Users/gal/.local/share/virtualenvs/storey-ptFrrr4H/lib/python3.8/site-packages/pandas/io/parquet.py", line 430, in to_parquet
    impl.write(
  File "/Users/gal/.local/share/virtualenvs/storey-ptFrrr4H/lib/python3.8/site-packages/pandas/io/parquet.py", line 204, in write
    self.api.parquet.write_table(
  File "/Users/gal/.local/share/virtualenvs/storey-ptFrrr4H/lib/python3.8/site-packages/pyarrow/parquet.py", line 2034, in write_table
    writer.write_table(table, row_group_size=row_group_size)
  File "/Users/gal/.local/share/virtualenvs/storey-ptFrrr4H/lib/python3.8/site-packages/pyarrow/parquet.py", line 701, in write_table
    self.writer.write_table(table, row_group_size=row_group_size)
  File "pyarrow/_parquet.pyx", line 1451, in pyarrow._parquet.ParquetWriter.write_table
  File "pyarrow/error.pxi", line 99, in pyarrow.lib.check_status
pyarrow.lib.ArrowInvalid: Casting from timestamp[ns] to timestamp[us] would lose data: 1580042497123456789

Expected Behavior

Up until pandas 1.4.4, it would print

2020-01-26 12:41:37.123456

and create the parquet file.

Installed Versions

INSTALLED VERSIONS

commit : 91111fd
python : 3.8.9.final.0
python-bits : 64
OS : Darwin
OS-release : 21.6.0
Version : Darwin Kernel Version 21.6.0: Mon Aug 22 20:17:10 PDT 2022; root:xnu-8020.140.49~2/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 1.5.1
numpy : 1.22.4
pytz : 2022.5
dateutil : 2.8.2
setuptools : 65.5.0
pip : 22.2.2
Cython : None
pytest : 5.4.3
hypothesis : None
sphinx : 3.5.4
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.3
IPython : 7.31.0
pandas_datareader: None
bs4 : 4.10.0
bottleneck : None
brotli : None
fastparquet : None
fsspec : 2022.10.0
gcsfs : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 6.0.1
pyreadstat : None
pyxlsb : None
s3fs : 0.6.0
scipy : None
snappy : None
sqlalchemy : 1.4.29
tables : None
tabulate : 0.8.9
xarray : None
xlrd : None
xlwt : None
zstandard : None
tzdata : None

[jbrockmendel]

This works now that we have non-nano support. Closing as complete.