According to the info on the promo it says BYOB. Now, earlier this evening I watch a video on YouTube by Stetson Doggett that his friend was able to buy a new phone and get the promo but you won't get the promo for the phone since the promos can't be combined. He pointed out that some of the new phones with the $200 promo would be a better deal and as the $15 off would only save you $180 in 12 months. Anyway, if his friend was able to get the $15 off by buying a new phone there should be no reason why you can't either.
Edit: I watched the video again where he mentioned about his friend, I guess he got a new number not a new phone.