B3 would be empty. This is the same as someone renaming any function or method and you asking "but how would the diff utility detect the rename?". The diff would show that the method used to exist and now doesn't. And it would show that the new method used not to exist and now does. The Excel-diff would show that B3 used to have a value and now doesn't. That B4 used to be blank and now has a value. It could show it in a rendering of the spreadsheet, even.

Your argument is that it's not possible. My argument is that it is possible. Nobody is arguing the strawmen positions that you claim I am arguing [it's a good idea, it's a way to build reliable software, everyone does it, etc.]