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  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:
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.
On the iPad portal with AT&T, it shows this as my plan: DOM Unlimited MB for 30 days of data per month.
It does not specify 3G or 4G. Furthermore, when smartphone owners upgraded from unlimited 3G to 4G, they were grandfathered in. To me, this indicates that AT&T will, for once, honor its committment to us and allow us to keep our unlimited plans with LTE. As for tethering...
Does 4G usually require a new microSIM?
This is the key question here.
I can personally attest to the SIM Swap working from iPad to iPad 2. I'm a grand-daddy 'unlimited' all you can eat Internet customer and I can say this; when I got the iPad 2, I simply popped out the microSIM from my iPad and put it into my iPad 2. And whalla - it worked flawlessly with my unlimited data plan.
Good morning, folks!
Checking in. I see that AT&T's login for prepaid 3G (or 4G?) is working again. So I'm going to give more specific instructions on what to do.
You can log in on the right side of the screen. Use the same login info that you use when you sign in on your iPad.
At the top of the screen, where your iPad's number is, you'll see a link for "edit user & payment information." Click on that, and the first thing you'll see at the top of the next screen is your device information. There's a button right below it that says "update device information." You can go ahead and change the IMEI and ICCID from there if you don't want to swap SIM cards.
I've been looking around online to get some other advice on what to do here. It seems that other folks outside of the Apple support communities are saying that it's a wise idea to let AT&T know what's going on first if you're going to swap SIM cards. But it looks like you can just as well change the information associated with your AT&T prepaid data account, too.
I agonized over the decision of whether to stay with AT&T in hopes of grandfathering in the unlimitted plan or switching to Verizon. In the end, I opted for Verizon. Here's why:
1. AT&T has (I believe illegally) changed their definition of "unlimitted" to "3GB@3G and unlimitted at dial-up speeds". I've never exceeded 3GB in a month but I can see it happening with LTE. It's just unacceptable to have a 4G connection drop to a couple hundred kbps or less.
2. AT&T's got great peak bandwidth numbers, but their latency is just inconsistent. Sometimes it's snappy, sometimes it's 10 seconds before a web image starts to load on a page. Verizon has lower peak rates, but on LTE their lowest speeds still exceed my needs. I'd rather have consistent 5mbps than inconsistent 300kbps-20mbps. I'd feel differently if I were downloading large files, but I'm talking about web browsing, email, video conferencing, etc.
3. Verizon LTE has the ANNOYING problem of not allowing voice and data at the same time (they do voice over 3G and data over LTE - see http://androidcommunity.com/verizon-rolling-out-voice-over-lte-in-2013-20120119). On a phone, this is a big deal, on an iPad, I couldn't care less. So while I'm waiting to jump to Verizon on my iPhone until they have VoLTE, this particular sticking point is moot here.
4. AT&T doesn't seem to be able to handle the bandwidth of their current 3G network. I have FIOS at home and it just screams. I had AT&T U-Verse at my last house and it (like my 3G experience with AT&T) has been a mix of horrible and great performance. As AT&T rolls out LTE, I don't think they'll be able to handle the load, at least for a while.
5. This is a big one that I haven't seen much talk about: Cook said that the Verizon iPad 4G will support global 3G roaming. That tells me that the VERIZON version will have both CDMA 3G (1xRTT) and GSM 3G (and if I'm reading the specs correctly, HSPA+). I'm not sure how they're doing it unless there's a SIM inside, but they said global 3G and that probably means this device has the least chance of being out of service. One point worth considering is that with a removable SIM there's at least the possibility you could use a prepaid data SIM while travelling out of country although I'm not sure if that works... I don't travel much, I just want maximum coverage...
6. Last point - I think both Verizon and AT&T are unrealistic when they cap LTE at anything less than 10GB. Part of the appeal of LTE is actually using it, and if it's 10x faster than 3G like they're claiming, they should expect people to be using higher bandwidth applications (video conferencing and HD video streaming come to mind). Hopefully this is just a temporary fear they're having about their networks being overloaded. Verizon in particular is ready for an unlimitted LTE plan with their 100% fiber back end. There's no limit on my home FIOS, there shouldn't be on my iPad either. I think Verizon is most likely of the two to realize this, and I for one would be willing to pay $99/month for unlimitted data if it included tethering and included my home FIOS (currently around $50/month). That's $1,200 a year; not a bad ROI for no up-front subsidy payout.
I recently read an article about AT&T's throttling, and the latest is apparently as follows:
The cap (at which the throttling begins) for a 3G user will increase from 2GB to 3GB.
Whereas the cap for a 4G user will be 5GB.
Calls to both AT&T and Apple indicate our grandfathered in data plans will remain intact (no one heard any news to the contrary).
HTH (hope this helps)
timothytripp, in reply to #3 on your list, you should realize that only on Verizon can you use LTE data while on a phone call. Sure, the phone call goes over 3G. But on AT&T, when you initiate a phone call, LTE is disabled and the entire connection downgrades to HSPA--both data and voice. So you may consider the dual radio use "annoying," but it's far more useful than not having LTE at all while on a phone call on AT&T.
This will be resolved on both carriers when each introduces VoLTE.