By the way, not allowing a change is stupid. We use different COGS accounts as a method of tracking COGS by product line.

If we re-use a part from one product line in another product then we need to be able to change the COGS for that part without affecting the past accounting.

Otherwise, what we will need to do is BEFORE ENTERING any new part, we have to think about whether or not it might be used in multiple products in the future, and if so, select the general COGS Materials account. If we are wrong, we are screwed.