No cell provider is doing "early" upgrades anymore. They are no longer subsidzing phone costs. They have started the NEXT program so that people that want to upgrade with each new phone model can, by trading in their current phone for a new model. If you pay off your NEXT program in full, the phone is yours to keep or sell if you desire.
With the Mobile share data plans, you receive a monthly discount for not having a subsidized phone contract. For most people, in the long run, the NEXT program is cheaper than getting a subsidized phone and losing the discount.