Was just doing some quick searching around on this myself, it looks like bringing phones between sprint and Verizon is the most difficult transfer you can do, and when I say most difficult I mean it can't be done.
A "good" phone on a cdma network generally has 2 choices. You can swap in a GSM SIM card and live on a GSM carrier while living with the issue if missing bands (so in some areas you will be 2g/3g restricted), and working with the SAME network on one of its prepaid MVNOs. Both sprint and verizon have been improving the BYOD life of its prepaid carriers by allowing its own phones to traverse over its own network on other MVNOs.
However like brothers that hate each other, the line is drawn between sprint and verizon. This might have to do with the fact that CDMA carriers can just be SIM unlocked and the carrier could care less, the SIM card handles the numbers on the GSM networks. But with CDMA the carriers would need to manage a shared database of each others phone identification numbers, and that is to much work.