AT&T iPhone upgrade after 20 months?

I have had an 8GB iPhone 3GS since the 20th of December 2011. As you can see, it's pretty old, and with it being only an 8GB, my storage is so low I can't even download anymore apps or music. My brother told me that for iPhones, AT&T lets you have a free upgrade (meaning you pay the price you would if you got a new contract) after 20 months. Is this true? Because I want to iPhone or at least 4S very soon, and I don't want to pay like 500 bucks. So can you really get a free upgrade for iPhones from AT&T after 20 months?

No, AT&T let's a customer upgrade at one year after the phone was activated. So yes, you can definitely upgrade. But you don't have to pay five hundred dollars. You pay what the new phone costs. But right now there's a deal at AT&T where you can upgrade your phone and the new phone only costs $99.