Easy. Just remove the SIM card and hold on to it, then install it in your new device. Your unlimited data plan is not tied to a specific device, it is tied to the SIM.
Just last week I bought an iPad2 to replace my iPad1. As soon as I unboxed, I swapped the SIM cards and voila, I had unlimited 3G on the iPad2. Didn't have to call AT&T to change anything.