Yes. You can use any phone that works with your current plan and ATT service.
There are exceptions... if you have a basic phone plan, and add a smartphone, it requires a data plan on the current plan.
If you are just swaping smartphones,it's as easy as moving SIM cards. If the card is the wrong size you can get a free replacement at an ATT corporate store.
(The following is included after all posts to save typing) We are customers too, NOT employees. Our answers are based on experience and reading content available on the website. Our answers are honest, but not always appreciated. If you posted personal information, please edit and remove.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.