I had a fascinating experience doing this last month...
I called Apple, and they originally told me they'd set me up with Best Buy so that I could get service within a week. I simply said that the phone was draining really rapidly while making calls, and I'd like the battery replacement, please. They didn't dive deep on the phone or it's condition, but immediately went to work getting me a replacement. They first set me up with Best Buy so could potentially get one within a week...
The day of my Best Buy appointment comes, and Best Buy calls me to inform me that, "Well, you'll need to leave your phone for 6-8 hours with us to replace the battery, because, we gotta do a bunch of testing. And, if we break the screen, we can't fix that, so you'll be without your phone until you can get a new one from Apple. And, iOS is really robust, so it might not actually fix anything on your phone." It was like hearing an Apple pseudo-science lecture given by a juvenile. So, I cancelled *that* appointment and did the 2-3 week wait time for an Apple Store appointment.
My Apple Store battery arrived within two weeks, I set up the appointment, and on the day of, aside from the usual cluster that is an Apple Store, I had a new battery, screen replacement, and phone cleaning within 90 minutes. Since then, my iPhone 6s has been behaving as new.
So much for iOS being "really robust."
