I assume you wish to transfer service between an iPad1/2 and a new iPad, the cards will fit, but AT&T has set them up not to work. You can however change service between an old iPad and a new iPad... There is an option to log in with an existing account, and update your new iPad with the old account info. So if you have paid for one account you can use it on two or more iPads, at different points in time.. It takes AT&T about 10 minutes or less to activate service on the new device.
Hope that helps...
If you happen to have an AT&T unlimited plan it will transfer between devices....
I can confirm that you can do this. I have an iPad (1) that has an unlimited data plan, when I got my new iPad, I called AT&T and they walked me through it, it really is very easy. After I completed the transfer AT&T told me I could throwaway my old sim card it would no longer work. I kept it, and found it useful to transfer the data plan back to the old iPad. I went through the steps and had the plan restored on the old iPad. No additional charges for this, and the plan has renewed. I'm not sure that AT&T would continue to permit you to do this daily, but a few times a month doesn't seem to upset them....