Something like that could probably be accomplished with a simple script. Without a script, I'm not sure.

It's generally bad practice to overwrite an item code in Sales Order entry. It messes up SO History and you get the issue you're reporting. A far better practice is to create a new line for the new item and then delete the replaced line.

We add a new line for the new item and then zero the original line - and add a comment as to why the SO was changed. We found there were too many instances of users changing line items and then issues arise around why was it changed and who changed it.