Getting a new (to me) device!

Hi! I use AT&T Prepaid and previously owned a Windows phone. After having gone through a couple, I have decided that I'd like an iPhone. I found a used and in good condition model on a website called Swappa. The listing states it's an AT&T device, so I am wondering if I am able to use the SIM card from my old Windows device and just pop it into the iPhone and resume service as usual. And since it's still the same pre paid plan, the rate should not increase, right?

Thanks for all answers!