When you first go into setting and select Cellular data you will get this window that pops up.
under where it says to upgrade your device on a current account, Click on the area that says "Existing iPad Customer?"
You will then be prompted to log into your ATT iPad account. Login with the info for your current account and magically your new 4G iPad will be the device active under your Unlimited Data Plan.
As you can see you have the option to keep the unlimited plan on the new iPad 4g.
How do do it via ATT website,
First you will need both to log onto AT&T’s website and transfer your account over to your new iPad. (If you don’t know or forgot your AT&T account password for your iPad, you can have one re-issued to you.) You will also need to know your new iPad’s IMEI and ICCID settings. These are both located in the General settings on the iPad under the About section.
This process should take effect instantly and will not change or affect any other information on the plan. It has been reported that original iPad owners that have been able to retain their unlimited data plan with AT&T will be able to keep this plan at the faster 4G LTE speeds. If you are unsure or just want to talk to an AT&T before doing the transfer, you can always call 1-800-331-0500 for assistance.
- Log on to www.att.com/ipadlanding to access your account information.
- Click on “Edit User & Payment Information” and then select “Update Device Information”.
- Enter in your new iPad’s IMEI and ICCID settings and click “Next”.