Upgrade question

I am trying to upgrade to a new phone. I chose the line that is eligible for upgrade, chose a phone, verified my trade in device and when I get to the cart it says “New Phone Line” . The number is the same as the one I’m upgrading. Is this adding a line with the same phone number or is this an actual upgrade ? I do not want to add another line. I just want to upgrade my actual phone and send back the trade in.