The only way to switch is ...
1. The old phone you wish to use has to be unlocked. If it isn't, then you need to call the carrier/service provider it was with before and have them unlock it.
2. If the phone uses a SIM (some do, some don't) they you will need a new SIM from the carrier/service provider you are changing to.
3. You will need to contact the new carrier/service provider and get the phone switched over to them. Only they can do that.