Change of Item No at FG Level

Hi

We r mfg part number with FG item code say "x" for which production order is released and subsequent transfers are made.At final inspection stage or say while finishing order sometimes we recognise that the item code "x" is not as per our quality which is supposed to be sold to OE customers. In such case Quality dept decide to sell it in the AM (After Market). Now here we want to change the item code to "y" since we can not use item code "x" which is meant for OE business only. I assume that As per excise law we cant transfer item code from "x" to "Y" after BSR is made.
Is there any procedure through which we can change item code. I know right from RM release material is released for item "X". but this is a practical case and i belive many of mfg .co r facing such issues and have solutions.

Thanx in advance

Pash