Surprisingly, they didn’t charge me! Two days ago I contacted them by DMing their support Twitter and asked if my plan supported 5G. They said yes, which is great, but that if it had been a while I would need a new SIM. When I said it had been four years, this was their response, verbatim: “In that case we should probably get you a new one! I would be happy to make that happen for you. It's going to be on us of course and I am happy to make this happen.”And I’m assuming since it’s T-Mobile, they probably charge you for it?
It was completely free and had free two-day express shipping. It came today and it’s their new R15 SIM card, the one recommended for 5G SA. Buuuuuut I know support can be kind of wack and it’s a crapshoot, so I may have just gotten lucky. But it was free for me!