Sprint & Boost Mobile contract?

Alright. You guys know how you can flash a Sprint phone to Boost? Or ATT to Qricket? Alright. I want an iPhone 6s (I've always had samsung and want something different). Now, If i were to get contract/installments on a Sprint phone, can I still flash it to Boost? Apparently Boost doesn't do contracts. How would it work?

Boost does not have a bring your own phone option.

Boost has iPhones and Sprint also has no contract plans.