Changing a customer currency which is different to company base currency

Changing a customer currency which is different to company base currency

I am trying to change the currency for a customer which is different to our company base currency. When I attempt to select the drop-down in the customer profile when editing, this is greyed out and I am unable to select anything else. Kindly advise as to why this may occur and how to change this. Does this depend on your package?