Well--from what I understand, you don't take the microSIM out of the old iPad and put it into the new one. I know Apple is perfectly fine with advising this--just do a search for it here at Apple and you'll find the information about how to swap out one microSIM for another. However, AT&T doesn't approve of this. According to AT&T, you're supposed to keep the microSIM card intact. Instead, you're supposed to update the device information associated with your prepaid 3G account.
One way to do this is here--you can call AT&T and speak to a representative. What you do is give the rep the IMEI number associated with your new iPad: http://appadvice.com/appnn/2011/03/howto-transferring-ipads-data-plan-ipad-2
I haven't tried that yet, but if anyone else here has (ie to upgrade from an iPad [1] to an iPad 2, or from one iPad to another), then let us know how that goes. 🙂
The other way to update your device's information is to go here:
https://dcp2.att.com/OEPNDClient/?wtSlotClick=1-0077VE-0-1&WT.svl=calltoaction
Two things to note at this point: Firstly, you can't go to the AT&T login page for the regular customers. (Or, I can't, anyway--because I don't have anything else from AT&T. For all I know, it's different if you've got other stuff for AT&T.)
Also: That link I posted was actually for the "add more prepaid data - shop now" link. I'm not sure why AT&T makes people hunt around for a login page, but this is the way I got to it.
Anyhoo--what you do is log onto AT&T with that login page and the same login info that you use on your current iPad. You should get a page that'll show you all your recent transactions--each time you renewed your subscription for the 3G. Near the top of the screen, you should see a button that'll allow you to view/edit your information--billing and device information.
If you click on the button, you should be able to edit the device information associated with your AT&T prepaid 3G account. I know one of the two things you'll have to change is the IMEI. I don't know what the other thing is because I can't get into my account at the moment...looks like AT&T is revamping the page for "the new iPad."
So at this point, I'm pretty sure I can keep my unlimited data plan. The next thing I'd be curious about is whether it would be 3G or if it would be 4G. I'm still definitely willing to take 3G, but still--just gotta wonder. 😝
I'm going to keep checking in with att.com/ipad. I was able to log in earlier today, while I was listening to the iPad keynote, and verify what I'd have to change with my account when I'd get my new iPad. When I can log in, I'll be sure to give more specific instructions on what to do.