Upgrade phone

I am needing to upgrade my current phone but every phone I want is currently out of stock with Xfinity.  I can purchase the phone directly from Samsung but I need to make sure the phone will be able to be used before I purchase it.  I am looking at the Samsung S21 Ultra.  If I can use, how would I go about activating it?  Would I just use the SIM card that is my current phone and put it in the new phone or would I need to be sent a new 5G SIM card?