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If the SIM is the same size, you can use your existing SIM.
Not all features may be available with older SIMs (not sure the technical reason behind this, but I digress), so if something doesn't work, you should switch to your new SIM.
T-Mobile generally suggests using a new SIM with every phone. I haven't changed SIMs in a few years, I don't think, and haven't had problems.
If you want to use your new SIM, you should be able to manage it at http://www.t-mobile.com/sim