Hi, This really depends on the promotion that is running.
there are 2 types of promotions that can exist:
1- a promotion from Sprint , most of the time its only for new customers and or adding a new line to an existing plan
2- a promotion from Best Buy ; you will have to read the details or ask at best buy to know if this is for new or for existing
We have sprint service and phones and the offer from best buy was too good to pass; they had 200 dollars discount on the new iphone 8 and 8+ and they also did a trade in for our existing phones. so we got the offer and upgraded 6 phones on our plan and reduced our monthly payment by almost a 100 dollars and we will own our new phones after 2 years of payments!
I hope this helps you out.