When I change base currency in IB, all the calculations to calculate tax in my currency (for me more than 30000 lines of calculation) is done for me, so I do not need to do the calculations myself.Base currency is irrelevant for tax purposes and also to trade assets in foreign currency. When you trade assets in currencies other than the currency of your tax regime then all profits and losses need to be translated to tax regime currency anyway, regardless of your base currency. And when you trade assets in other currencies then you need to have cash available in that currency or utilize margin loans provided by a broker. That applies even if your base currency is USD and you trade USD denominated assets. It is entirely possible that you may have usd base currency but not a single usd in your account but have all your cash in other currencies.
When I change base currency in IB, all the calculations to calculate tax in my currency (for me more than 30000 lines of calculation) is done for me, so I do not need to do the calculations myself.
I guarantee you that is not irrelevant.
That is great for you. However, they do not offer that service for me unfortunately.IB must do it for residents of certain tax regimes regardless of the base currency. It's a regulatory requirement. For example I can choose whatever base currency I want. After the end of each tax year with me being based in Canada IB must furnish a tax record of all trades with pnl converted to Canadian dollar, no matter my base currency. They are even forced to use the BoC official fx conversion rates.