You might need a new SIM and you might not. I say that because when I upgraded my wife's phone (took her from a 6S to a 12), I took the SIM out of her old phone and put it into her new one and she has 5G. And I think that is because the plan we are on has 5G included. I have heard of people getting a new SIM but the older plan they are on doesn't include 5G and they never saw it, and when they changed plans which included 5G, they suddenly have it. It may have more to do with the backend with AT&T on how it looks at the plans. I'd be willing to bet that once all these older plans have 5G added to the accounts on the backend, you will get a text notifying you that you have it and you will probably see it.