You can change the Bill Details for a Bill of Materials product at any time. Irrespective of whether the Bill has a quantity. The product that is manufactured is essentially the same, so Bill Details changes can be made.
However, if a Bill of Materials product has a BOM in progress, the work in progress Bill of Materials product will be created using the original Bill Details list of product components. If any BOM is in progress, complete it if needed OR it may need to be cancelled and re-generated.
You can edit the product code directly from the Product Masterfile, Bill Details tab.
Go to Products, Masterfiles, Products, Maintain, Bill Details tab.
in the Bill Details list, click into the Product Code column for the code you want to update.
When updated, click Accept (F9) to save the changes.
Repeat the process for any other Bill of Materials.