Let's assume that my base currency is in EUR and that I also have USD in the same account. How do I change my base to USD without converting the EUR? Does IB's system give you an option? This is the one feature that the paper trader does not allow you to test out.
I don't think you can change your base currency. Everything is referenced to this currency for accounting purposes. But you can hold securities or cash in any currency and they are valued at each day end based on prevailing exchange rates.
The base currency can be changed anytime in your Account Management. It is merely how your balances are displayed and is independent from your actual cash positions. None of your cash will be converted.