Apple recommends that you backup prior to switching it to a new phone. So you back up. New phone arrives. You try to transfer your old phone to your new by placing in proximity. But it tells you that you cannot, because the iOS build on your old phone is newer than that on the phone you just received. So you have no option but to set the new phone up as a new. Then you must update it to the newest version of iOS (new builds have come out since it left the factory). Then you can restore from backup. The only issue is all of the time wasted setting up the new iPhone, just to then restore from a backup.