Non smart phone to smartphone upgrade

First of all, searched and couldn't find the exact anwer to my question.

I have a Pantech Ease, which is not a smartphone. I am looking to upgrade so an Iphone 3GS (yes I know the 4G is out but I don't want to spend that much money). We have the family plan with my phone and the kids (2) each with an Iphone 3GS. We have the old unlimited data plan, not the new one that came out awhile back.

My question is: If I buy a slightly used ATT Iphone 3GS will I be able to transfer the SIM from my Pantech and not mess up my unlimited data plan. I would be buying the phone from an individual and not from ATT. I don't want to buy it and then go in there and have them tell me that it changed my data plan.

Any responses would be greatly appreciated.

---

› See More: Non smart phone to smartphone upgrade

If you add any iPhone, it will be detected and the appropriate data plan will be added. Since you did not have an unlimited smartphone data plan before, you cannot get it now.