I currently have an iPad Mini retina LTE that I bought from T-Mobile in April. Has the 200 MB/mo free plan plus the 1GB/mo plan (free til the end of the year). If I were to buy another iPad Mini Retina with more storage or even a LTE iPad Air, but a Verizon or AT&T version, can I just pop in my T-Mobile sim and use it? Or are the Verizon and AT&T iPads locked to the carriers? Thanks!