As you know, all items have ID codes. Those ID codes are what is saved in a save file, rather than the name of the item we see. If you change the ID code of an item then when the save file is loaded it looks for an item that doesn’t exist and anytime you go near where that item was it will throw an error. However, you can set up transition code that converts the item with the old ID code to the new ID code (apparently). That way it will convert all the items with the old ID to the new one, so you don’t throw errors with that save.
However, since the game only loads data near where you are, if you try to transition something common, like a drink can, you would have to walk near every single existing drink can in the game for them all to transition, otherwise it will still throw errors when you walk near one that hasn’t once the transition code has been removed.
All that said, could someone give a quick example of what that transition code would be?