The reason you need to map a Constituent Code in your template, even if you are only updating Constituents that already have a Constituent Code in their record in RE is because you have a Business Rule in your RE that requires a Constituent Code. Importacular cannot overwrite your RE Business Rules and when starting an Import, Importacular checks those Business Rule first. It will look to see if Constituent Codes are required and if no Constituent Codes are mapped, it will throw an exception. At this stage it has not looked to see if records matched in your database or if they too have Constituent Codes.
You can either add the Constituent Code into your template, or remove the Business Rule in your RE