Can I buy a used iPhone 4s and use it with my AT&T plan?

I currently have a simple non-smartphone (is that even a word?) If I was to buy a used iPhone 4s could I take it into AT&T and have them put a sim card in it for me and use my current plan? How could I switch to an iPhone without an upgrade?

Yes it can be done! Its just that you have to buy this phone on Ebay, cregslist, or maybe a friend that upgraded to the iphone 5.

You could buy the phone off amazon too. But REMEMBER: phones cost MORE when you buy them without a contract. Because the original price of the phone is probably about 700 bucks. It helps when you buy the phone now since the 5th one is out so you should be able to get a nice 16gb one for like 350? Around there. The phone MUST be Unlocked!

Also remember, that data plan is required for iphone so you will also have to pay extra (20 or 30 bucks) every month.