Changing phone, but keeping the same account

    this may seem like a silly question but this would be my first time changing phones using t-mobile


    I have a lumia 635 that I bought from the Microsoft store (so I registered it and inserted the sim myself so I get use t-mobile's [first-time only] 30 dollar plan). it is great but my brother needs a phone since his zte phone is starting to fail him (phone-wise and service-wise. I also noticed that there is the 640 coming out, which I was planning to get via Microsoft store.


    the question is how does the phone "upgrade" work? do I take out the sim from my old phone and put it in the new phone or do I set up the new phone, like the old one, with my existing account (which may give me an option to add new phone and/or replace the old phone)?


    thank you

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