Originally Posted by
dharper57 I do not know yet but my husband is going to be doing that with my iphone 5s since I upgraded to a 6 but I will tell you we did this with a 4s from his 3s and it was not compatable but we called AT&T and they shipped a new sim within two days and we were fine no extra charge or anything but I am hoping it's a simple as swapping sim cards.. Just FYI
I believe the reason why AT&T needed to send you a new sim for the 4S was because the 3GS used a full size sim and the 4S uses a micro sim which is smaller. Swapping sims between a 6 and 5S will be no problem size-wise since both use the same newer nano-sim cards.