unlocked iphone

This might be a silly question, but everything I see online regarding using an unlocked phone with tmobile is about bringing your own device to start new service. It isn't very clear on using an unlocked phone for an existing customer. My fiance has tmobile and I have at&t. I submitted an unlock request for an iphone 6+. Once it goes through, can we just take the sim card from her tmobile iphone and put it in the unlocked 6s+ (they both use the nano sim) or will she have to get a new sim card?

We got the phone unlocked and didn't need a new sim card, for anyone looking for this info.