I bought an AT&T GSM iPhone 6s Plus full price via the Apple Store App. It will arrive unlocked and without a sim card. I have an existing iPhone 6 Plus which I will backup with encryption. I was told by AT&T support that I put the sim card into the new iPhone after I restore the new iPhone from a backup. I wanted to reach out to find out if that's indeed what I should do. I had thought that I should put the sim card in before backup, but I guess not.
I doubt it. It should come with an AT&T SIM card. Once you activate the phone, the new sim card takes your phone number and disables your old sim card. That's what AT&T charges $15 activation fee for.
That's correct. It will have a SIM card already programmed. You can either activate with that one or take it out and use your old one. If you decide to use the new one, your old one will automatically be deactivated.
I suggest that you remove the SIM in the new phone and put yours in before you turn on your new phone. They should not charge you any activation charge what so ever.
My current phone number is on a Iphone3 (broke my 5 last month). So, can not take that sim out an switch I guess.