You wouldn't have to, just trade in your phone like I did my 6.I'm impressed, but for me, there is no reason to upgrade from my perfectly functioning 6s Plus. I wasn't planning on upgrading anyway, but seeing the 8 doesn't compel me in any way to upgrade. I know that everything about the 8 is probably better than my 6s Plus, but I have no issues with the performance, camera, screen, battery life, or design so I really don't have a reason to fork over $800 for an 8 Plus.
Really good phones though, and I would definitely upgrade if my 6s Plus wasn't still going strong.
My payment for the 256 8+ is 17.50 a month...well worth it.