While you can change the currency in which positions are denominated in for statement reporting purposes through selection of a Base Currency, you cannot change the denomination for trading, clearing, settlement or profit/loss purposes. The currency in which a product is denominated for trading, clearing and settlement purposes is determined by the product specifications of the listing exchange and is not subject to election by IBKR or the client.
Note: TWS offers a feature which will display position values in your base currency. All position and P&L values are recalculated to display in your base currency, and are shown in italics to identify them as having been converted from the local currency to your base currency. However, as noted above, this will not change the denomination for trading, clearing, settlement or profit/loss purposes and is strictly informational.