Hi,
Yes, you can do that with a mass action (menu System>Mass actions) with an action "update the values" on the column "price_currency_id" with the value corresponding to the id of the currency (2 for USD).
Note that the limitation area will restrict the change to the 500 first products when you run the mass action so you'll have to adapt the start/limit fields and run the mass action several times to cover all the products.
Otherwise, you could just run a MySQL query in your database via your phpmyadmin. It's quite an easy query:
UPDATE #__hikashop_price SET price_currency_id=2;
(you need to replace #__ by your table prefix)