...can somebody clarify this conundrum for me (I'm probably being stupid!!

)?!
When I went on my Apple Store App to the IUP, to see if it would be possible to exchange my iPhone 8 Plus to an iPhone X (I am NOT looking to change my iPhone 8 Plus - just wanted to see if it was actually possible!!), it stated that in order to do this, I would have to pay the remaining balance of my current IUP, which is around £453), and then hand back the phone (when at the store), which would then enable to me to reserve an X, for collection (again - I am not looking to change).
Unless I am missing something or have completely misunderstood this, why would I have to pay off the balance AND hand back the phone??

Surely,paying off the balance means the phone is mine to keep, no? Why does it say to pay the balance AND also hand back the phone?!