Welcome to the QuickBooks Community. I can see the benefit of being able to set USD as your default currency and I'll be glad to point you towards the right path.
In order to change the currency on the account, click on the Gear icon > Account and Settings > Advanced > from here you can change your default currency. Make sure your changes are saved so they reflect on your account for transactions going forward.
Give this a try and let me know how you make out. I'll be a message away in case you need anything else.