Generally, when you change guaranies at a bank office, they will already know their clients and you will only need to identify yourself as a customer of your bank.
If you change guaranies at a travel money provider´s office, on the street or at the airport, you will need to show a valid and current identity document (resident´s ID card (DNI or NIE) or passport in Spain). Have in mind that your driver's ID will not be accepted as a valid ID when changing Euros to guaranies.