If an improvement creates no significant change in a product’s performance, then it is a(n) ? a0 design improvement. (there are no answer choices)

You would imagine it would be a design improvement since the performance hasn't been upgraded. For example, the new iPhone XS Max has a larger screen than the iPhone X did when it first came out, but overall performance was not increased; it still had the same processor and graphics card, but just a larger screen and higher pixel per inch ratio. If there is no change in product performance AND there is no change in the design, it essentially the same product.