Actually it really depends on who you talk to. I had the same issue with my 15. They sent me an iPhone which had a worse screen. I called them, and told them the problem. Sent the phone they sent me, and just kept my original one.If you get a express replacement sent out, it is not possible to change your mind and keep your old one instead.
I have tried it myself and Apple said that there is no way to cancel the process. I had to send back the old one or else they would’ve charged me full price for the replacement.