def modify_cc_prime_mover_code(df, gens_860):
    """Change combined cycle prime movers from CA and CT to CC.

    The heat rate of combined cycle plants that aren't included in PUDL heat rate by
    unit should probably be done with the combustion and steam turbines combined. This
    modifies the prime mover code of those two generator types so that they match. It
    doesn't touch the CS code, which is for single shaft combined units.

    Parameters
    ----------
    df : dataframe
        A dataframe with columns prime_mover_code, and plant_id_eia.
    gens_860 : dataframe
        EIA860 dataframe with technology_description, unit_id_pudl, plant_id_eia
        columns.

    Returns
    -------
    dataframe
        Modified 923 dataframe where prime mover codes at CC generators that don't have
        a PUDL unit id are modified from CA and CT to CC.
    """
    cc_without_pudl_id = gens_860.loc[
        (gens_860["unit_id_pudl"].isnull())
        & (gens_860["technology_description"] == "Natural Gas Fired Combined Cycle"),
        "plant_id_eia",
    ]
    df.loc[
        (df["plant_id_eia"].isin(cc_without_pudl_id))
        & (df["prime_mover_code"].isin(["CA", "CT"])),
        "prime_mover_code",
    ] = "CC"

    return df