I am with T-Mobile and want to switch to AT&T. I assume I can just walk into a store and tell them I want a Mobile Share Value Plan? I haven't decided if I will go with the 4 or 6 gig plan. Will they just pop a new SIM in and then my number will eventually port over? I have a Nexus 5. Is there anything special I need to do to get MMS / LTE working?