Can I use an unlocked phone with a 10 cents per-minute plan?

My father currently has a Samsung Galaxy Express 3 phone on a 10 cents per-minute plan. He wants to purchase an unlocked Samsung Galaxy S10 from Amazon for a discount and activate it on his current plan. Best Buy also has a discount but the sales associate there they told me that my Dad needs to be on a post-paid plan to receive their discount otherwise the phone itself will cost an extra $200, which I don't understand why. Anyway, my question is can purchase the unlocked phone from Amazon and bring to an AT&T store for activation without any additional fees (excluding a new sim card)?