It is my understanding that all iPads come "unlocked" regarding cellular systems. Also, I hear that LTE iPads can accept any LTE sim, per FTC rules. So what is the difference between an ATT iPad and a T-mobile iPad for instance, other than having an ATT sim pre-installed by Apple? If I buy a new "ATT" iPad from apple, is there anything to stop me from changing the sim to another carrier when it arrives at my door? Thanks