The "Banking and General Ledger => GL Accounts => Edit Bank Account" does not bear the Currency details.
The "Banking and General Ledger => Bank Accounts => Edit" does not allow the changing of Currency once set. This can be changed in the database directly only. Take a backup, correct it in the sql and upload and restore it. This should be done only when you know that no transactions exist for the said Bank Account as yet.
Make sure that the Home Currency is properly set in the Setup => "Company Setup".