A:Answer Yes, in essence, they transfer your old phone's SIM card to this new iPhone. Then, you can keep your same plan without adding a line. You can also see if Best Buy offers you a trade-in value for your old phone to save some money.
Be advised, though. If your previous phone had a LTE SIM card, there could be a cost to upgrade that SIM card to 5G which is needed for this new iPhone.