So, you CAN upgrade to a new subsidized phone without losing the plan?
https://www.sprint.com/en/shop/offers/free-unlimited.html?ECID=vanity:1yearfree
When can I upgrade to a new phone on this promotion?
Upgrades for the lines activated on the Free Unlimited plan will be allowed after 4 months of service. At that time, return to sprint.com and log in to your account to purchase/lease your new phone. When it arrives, visit Activate your device to activate the new phone. Any upgrades prior to the 4 months or in a retail location will require a plan change to the Unlimited data, talk and text plan which starts at $60 per month after an AutoPay discount. Unfortunately, you’ll lose the remaining free months in your year of free service if you upgrade early or purchase/lease your new phone in a retail location.