Creating a new family plan

My wife and I are both currently on our parents family plans (both T-Mobile). I’m on magenta max, wife is on something different I believe.

Wife needs a new phone and we thought this would be a good time to start our own contract. What’s the best way to go about porting this over? I’d like to still be on the magenta max plan. Can I go into a store and do it or do I have to order it online and process the trade in online?