I recently got laid off and I want to sell my 3rd Generation iPad until I get another job. However, I do have the $29.99 unlimited data plan that I got with my first iPad. I don't want to lose this, but I DO want to sell my iPad and disconnect it from my iPad. I know that sounds crazy, but my wife is probably going to get a mini with Cellular soon. I called AT&T and they really didn't have any idea how to do this.
I really doubt AT&T will allow you to transfer the plan to a different iPad. Especially if you don't do the transfer at the same time you stop using the plan on one iPad. In any case this is a question between you and AT&T.