Want to switch off JoD, what are my options?

I joined Jump on Demand during the iPhone promo, and currently have an iPhone 6s+ 64GB. I want to move to a prepaid plan (TMo $30 plan, Project Fi, or Cricket), and am wondering what my best options are. I plan on switching to the Nexus 6P, so I want to get rid of my iPhone.

I've read some users here "jump" to another phone, then return it and can leave the plan at no cost. Can I do this? Would it be better to just buy out the phone and sell it?