How to upgrade your current At&t phone to a used iPhone?

So I have this silly cheap slidy phone that has a normal size SIM card and I really need a smartphone. It's on a cheap family plan with At&t, just unlimited calling and texts/data added on when I use it (I rarely ever do).

I don't want to pay for a brand new iPhone, nor can I afford to pay a large monthly fee with a two year contract for a "free" smart phone with a new plan. I would not like to change the plan I'm using, just the phone.

If I bought a used At&t iPhone (4th gen or newer) off of craigslist or eBay, how would I go about switching my plan onto it? I did some research and came across a bunch of nonsense involving cutting the SIM down to size, but is there an easier way to do it? What if the iPhone does not use a SIM card? Sorry if the question is slightly confusing but I'm completely befuddled.

The best thing is to get a used "AT&T" iPhone so you don' have any compatibility problems, then just get a new smaller SIM from a AT&T store, you might be able to get it free if you ask.

Use eBay or Amazon, so many iPhones sold on craigslist are stolen and "blacklisted" and just a waste of your money. I buy All my smartphones from eBay, then sell the old ones, but I have T-mobile not AT&T.

Well since the Sim card you have is the normal sized one an iPhone 3GS or lower would be a option
But if you wanted something above a iPhone 4 then you will have to do a size reduction on the sim card. You can get a kit from eBay, Amazon, etc.
There are also other options such as finding an android phone with the same size simtray.