Will my iphone 5 SIM card work in iPhone 5s?

I want a new phone but I'm only a year into my 2 year contract and I don't really want to pay off AND buy another phone, so if I get a 5s will everything work the same and will at&t do anything about it or is the contract about the billing rather than the phone?

Both the Iphone 5 and 5s should use the same nano sim card since you are on att. If you buy another phone not using an upgrade you should be able to just put it in the new phone and you should be good to go as long as it uses the nano sim card.

If both phones are on AT&T then yes it would work as both phones use nanoSIMs