Unfortunately, it could. Numbers you have entered into forms are presumed to be in the currency set as your base currency unless, for example, you are entering a sales invoice for a customer in a foreign currency. Then, the numbers are presumed to be in the customer’s currency. (This is why the currency shows in transactions.) Or, if you make a payment from a USD-denominated bank account and switch base currency to GBP, the payment remains in USD because the bank account was not changed.
I you change the base currency now, none of the numbers you entered will change. But numbers that were converted to base currency for balance-sheet purposes will change, because they will now be presented in the new base currency according to the exchange rate in effect. In some cases, you could end up with something you entered as 50 USD now being 50 GBP. But numbers like Accounts receivable balances for individual customers won’t change, because they were always in the customers’ currencies. Yet the overall Accounts receivable balance will change because the customer subaccounts will be converted to a different currency. Complex, eh?
You will need to be very meticulous in your conversion. Be sure to make a backup before you start. You might want to re-enter all your transactions in a new business with the correct base currency setting. At a minimum, you will need to carefully check every account balance, scan through all transaction lists, and so forth.
Coincidentally, there is another discussion going on right now on this very subject: Employee clearing account. You might find it useful.