How do I copy contents from one column to another while using .apply() in pandas?

Question

I have a DataFrame with 2 columns total_open_amount and invoice_currency.

invoice_currency has

USD    45011
CAD     3828
Name: invoice_currency, dtype: int64

And I want to convert all the CAD to USD from the total_open_amount column wrt to invoice_currency with an exchange rate of 1 CAD = 0.7USD and store them in a separate column.

My code:

df_data['converted_usd'] = df_data['total_open_amount'].where(df_data['invoice_currency']=='CAD')
df_data['converted_usd']= df_data['converted_usd'].apply(lambda x: x*0.7)
df_data['converted_usd']

output:

0            NaN
1            NaN
2            NaN
3        2309.79
4            NaN
          ...   
49995        NaN
49996        NaN
49997        NaN
49998        NaN
49999        NaN
Name: converted_usd, Length: 48839, dtype: float64

I was able to fill the new column with CAD values converted but how do I fill the rest of the USD values now?