Under the Consumer Laws I mentioned, manufacturers have more leeway, but a business cannot require you to deal with the manufacturer.

Apple is required, and does if you point out their obligations, to provide a new device if you purchased a new device.

As phones are also guaranteed a "reasonable lifetime" of 5 years, this has led to replacing 4s with 5s and the like.

However, as you mention ATT, you might be American, in which case, almost none of these guarantees applies to you.