If you want to switch, you should contact Sprint as to whether or not you can use your current devices.

There will definitely be fees associated with switching. You will be responsible for an service charges plus any remaining balance on your your devices or, if you are under a 2 year contract, any early termination fees. If you switch, Verizon will send you a final bill that reflects all balances due. However, if you have a plan that has it, turn on Safety Mode. This will not charge you overages but WILL slow your mobile data speeds considerably. You might also want to look into one of the Unlimited options. Or, use WiFi more and be more cognizant of your mobile data usage to avoid overage fees.