Using the AT&T sim already activated should not be an issue, I have a 3g AT&T sim from my iPad 2 which I am keeping so no worries about activating. But can you then swap back and forth between the 3g AT&T sim & Verizon LTE sim? I really can't see why not...but seeing if anyone is doing this. If this works well, the options seem great to own the Verizon Model, My AT&T should be faster on HSPA+ and I have Verizon on my iPhone so I can use the 4G and hotspot if needed on Verizon. Of course if they for some reason ever block us from using an AT&T sim in the Verizon model that would be pretty bad.