If you already have transactions in your database for an item code, you really can't go back and change the item code (or customer code, etc.). That item code is referenced in hundreds - maybe even thousands - of places. Attempting to change one of those item codes is a Bad Idea.
A much better solution is to create a new item code, list this new item code as an Alternative Item to the old one, and move the stock over to the new item code. If the Automatic Availability Check is functioning properly (heh), it should tell order-entry clerks or customer service reps that the old item is out, and that they can check Alternative Items to see what new ones are available (and how well they match).
Karen, I tend to agree with Patrick.
however I do want to inform you about a solution partner that does what you need, if this is something you do often, it's probably worth checking out.
I believe they are not certified by SAP yet, but they are in the process, please check with them.
I am attaching their brochure.