If there are no transactions on the account, this indicates that you have an analysis code setup to target the account.
You will need to find the analysis code that is targeting the code using the following method against either your sales or purchase ledger codes the nominal type you have created.
Go to Debtors, Analysis (ribbon menu) or Sales Ledger, Records, Analysis (classic/module) menu for sales ledger codes
Go to Creditors, Analysis (ribbon menu) or Purchase Ledger, Records, Analysis (classic/module menu) for purchase ledger codes
Find the analysis code that is targeting your new nominal for either debit or credit.
You will find that the analysis code will also be in the wrong currency
Select the Edit button and update the currency on the analysis code
Once you have done this, you will then be able to update the currency on the nominal record.