https://www.theverge.com/circuitbreaker/2017/9/18/16326842/sprint-iphone-8-preorder-deal-flex-lease-discount-trade-in-switching-customers

Sprint has announced that it’s upgrading its iPhone 8 preorder deal, with the carrier now offering an 18-month lease for free when you trade in an existing high-end device instead of the original 50 percent off lease it had originally offered. As is the case with all of Sprint’s leases, after the 18-month period expires, customers will then be able to return the phone and upgrade to a new device, or pay out six more months at the regular $29.17 / month price and own the phone.

It’s important to note that the discounted $0 / month price is only valid if you’re activating a new line and trading in an iPhone 7, iPhone 7 Plus, Samsung Galaxy S8, S8 Plus, or Note 8, which means that existing Sprint customers won’t get the newly discounted price. Customers already on Sprint or new customers trading in an older device will still only get the same 50 percent off as before.