Changing Currenncy

Scenario :

Hi All, Am using CRM 2011,

The base currency is Euro, we have multiple transaction currencies as well in CRM.

Now while closing an opportunity (as won or lost) we have a managed field with type as currency and there are multiple dependencies on the same.

What we need?

By default the managed field is pre-populated with a Euro.

To change the currency from Euro to USD (symbol) when the dialog box appears while closing the opportunity.

We can't delete the field as well.