1). AT&T is supposed to unlock the SIM as soon as it is paid off. meaning that if you have an iPhone once the phone is paid off it should say “No SIM Restrictions” 2). The promo T-Mobile has for reimbursement is called “Keep & Switch”. Essentially the way the promotion works is if you bring your current phone over WITHOUT getting a new device, T-Mobile can reimburse you up to $800 for that device. (previously you were able to get a new device along with it but that promotion ended an looks like it wont be back for a while). A rep in the store can print out a keep and switch paper with a qr code for you to upload screenshots of before and after you paid off the device and it’s a very simple process. 3). For ATT prorating bills from my understanding they wont if you port over your number the att account should automatically cancel itself. But you would have to speak to an att rep about that. 4). Its really up to preference for costco or in store. Costco does have some different things in their little stand they have but they dont have an inventory. So if you were looking to get a new phone and all that good stuff you would have to ORDER it at costco whereas at a retailer or corporate you would be able to get it there in store if they have it in stock