Does it cost to add a used iPhone to my line at Sprint?

I currently have a non-smart phone through Sprint and was looking to get an iPhone. Right now I'm unable to get one cheap with my plan. I saw a lot of good priced used Sprint iPhones here in town, I didn't know if I could bring one in and just flash my phone over to a used iPhone instead… Didn't know if there are any additional fees? Or if it is as easy as that. Anyone know? Sprint is closed right now, so just looking to get some answers before I head down there this week.

You will have to purchase a data plan.

Call sprint