From what I found on ggle on ib you still have a base currency for your wallet but possibility for next market day to change your base currency
When you but EUR/USD pair, you have possibility to close your position with option to close "non base currency positions" assuming that as this moment your base currency was changed to USD if you want your EUR to become USD? I'm not very sure with that I need to try by myself, but I guess this is something like that behind professional currency cambists?
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Exemple given here
Converting Currency Balances (EUR/USD) Interactive Brokers / Lynx Trader
When buying stocks in USD market when your base currency is EUR makes ib open for you a currency coverage position opposite.