Transferring unlimited data to iPad Air 2
I restored my new iPad Air 2 with the back-up of my current iPad Air.
When I picked the cell data option I picked ATT and logged in. It then asked me how I wanted to buy data and the third item on the list was to "transfer data (or account, I forget which) from another device."
I picked that and it found the ATT data plan on my current iPad and it disappeared from that one and showed up, after a bit, on the new iPad Air 2 transferred to the new SIM. It took some time to initialize-- a couple of minutes.
For some reason I could not check the account info on the iPad itself (I was given a phone number to call) but I went to the ATT iPad landing page, logged in, and could verify that the phone number on the iPad Air 2 matched that on the Web site and the data type was still the "unlimited data" option.
The process was easy and all is well.