How to change order currency in cart/checkout page?
You cannot directly manipulate the order total like this, as it gets recalculated based on its line items' prices each cart refresh. (That happens on every pageload in checkout and however you've configured it for other routes.)

You should look into the Commerce Multicurrency module instead or else configure your pricing rules / otherwise hook into product sell price calculation to change the currency on the order items (and shipping if present). If those are all the same target currency, the order total will use it, too.