Switch from premium account to prepaid?

Sure you will have to pay off all your devices in full in order to switch. 

In the past, you had to do a migration in an AT&T corporate store. You may have to find an authorized retailer willing to do that for you. AT&T claims that they can do it over the phone but every time someone calls they're told to go to a store instead. 

You could also pay off your phones, unlock them, and go to an AT&T MVNO like Cricket