It was like when t mobile bought metro pcs. Metro was a cdma carrier and t mobile a gsm carrier. They kept metro’s network up for a while and gradually migrated their customers over to gsm phones. From what I hear sprint’s iPhones when unlocked are universal phones so that would be an easy fix as they could just unlock the iPhones so they work on a gsm network.
It will take years to merge the networks. I remember when sprint bought nextel, and they kept the networks seperate for many many years - far too long in all honesty but I mention that just to point out, a merger in many respects is not a simple flip of a switch.