It's been awhile since I bought an iPhone (3G), so excuse the obvious question: If I buy the 4S online via Apple.com, and want to keep the same provider (AT&T) and plan, how do I activate the phone once I receive it? Do I need to go to an Applestore or AT&T store, or can I activate it from home? Less obvious question: Same scenario, except I want to change from AT&T to Verizon. How/where do I activate the phone? TIA.