If there are no transactions on the account, an analysis code is targeting it. Find the analysis code using the following method, depending on your ledger type:
Sales ledger: Click Debtors > Analysis (ribbon menu) or Sales Ledger > Records > Analysis (classic/module menu).
Purchase ledger: Click Creditors > Analysis (ribbon menu) or Purchase Ledger > Records > Analysis (classic/module menu).
Find the analysis code that is targeting your new nominal for either debit or credit.
The analysis code will also be in the wrong currency.
Click Edit and update the currency on the analysis code.
Once updated, you can change the currency on the nominal record.