It all depends on what app you are using for contacts. If you are using a carrier contact app or a phone manufacturer app you are always faced with this issue if you want to change carrier or phone company. If you use Google contacts or any cloud based app that is not carrier or phone manufacturer based, your contacts are always there and you are free to move to another phone or carrier company with no effect on your contacts. It is easy to import contacts from any other application into Google Contacts before you switch to avoid losing contacts in a switch.