df = pd.DataFrame(
    {
        "a": [1, 2, None],
        "b": ["aaa", None, "ccc"],
        "c": [None, 'B', 'C'],
    }
)
df['a'] = df['a'].astype('Int64') # pd.Int64Dtype()に変換
df['b'] = df['b'].astype('string') # pd.StringDtype()に変換
df['c'] = df['c'].astype(pd.CategoricalDtype()) # pd.CategoricalDtype()に変換(Noneはpd.NAに変換されない)
df