Tax Group in Receivables 11i

There is one tax group suppose 'ABC10' containing one Tax code 'TEST10'  VAT having start date as 01-aug-12.

If we enddate the above Tax code 'TEST10' on 31-Aug-2012 and create a new tax code with same name exactly as 'TEST10' with effective start date as 01-Sep-12.

While creating Transaction in receivable in sep month if we use the above tax group which is having the Tax code 'TEST10' which is end dated why still it is calculating tax using that tax code.