Sprint is a CDMA carrier. I'm not sure if Metro pcs is CDMA, or GSM.
That's your first problem. You second is that carriers like to lock their phones so you can't use them on other carriers. If you are out of contract with Sprint they might give you a code or something to unlock your phone, and if Metro is a CDMA carrier then there is a chance you can use your current phone.
If I were you, I would just get a new phone. It's not like you are going to same any money on your contract if you don't sign a new contract and get a new phone.