Update currencyCode for order

[prasmalla]

Is your feature request related to a problem? Please describe.
It is not possible to change the currencyCode for an order once initiated. In a multi-currency setup, removing the cart contents and adding it back under different currency only sets the price correctly but currency for the order is not updated.

Describe the solution you'd like
When a user switches to a different currency, the currencyCode for the order should be updated. If the cart has items added previously in a different currency, it should update the prices appropriately.