It is important to note that changing your Base Currency does not serve to convert existing cash balances or change the denomination of a security into any other currency.  Cash balances will remain in their currency of denomination unless converted via IB's IdealPro (for balances of at least USD 25,000 or equivalent) or odd lot (for balances below USD 25,000 or equivalent) Forex venues.  Similarly, the denomination of securities and commodities positions cannot be changed as this is determined by the listing exchange.