I have an iPhone 6s upgrading to a iPhone 7 plus my upgrading my I go to US cellular I'm just wondering that if I have to buy a SIM card?

I have an iPhone 6s upgrading to a iPhone 7 plus my upgrading my I go to US cellular I'm just wondering that if I have to buy a SIM card? - 1

Not always though, it's better to buy a new sim card. You can also contact the Carrier company and ask one of their employees if you like