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Contact T-Mobile or go to a store for fastest service. They should be able to get you set up with the Sim card and you should be able to continue using the same number and plan you got however you may have to re-pay if you've already pre-paid. Best bet is to go on the last day of your pre-pay cycle.
Yes you can. I'm on the same grandfathered plan you are and I was able to transfer my old plan, number and balance to a new different sized SIM card for my new phone. You need to buy a new unregistered SIM card in the correct size from a T-Mobile store or another source. Then call 611 from your old phone and give them your new SIM card number. They said it could take up to two hours but I was up and running on my new smart phone in five minutes.