Changing phones without changing anything else?

Ok. so I have a T-Mobile family plan that is from 2006-2008. I have a RAZER, but I want to buy the 50 dollar sidekick. How can I do this without having to renew my plan, or change my phone number? Is it possible to just put in my SIM card in any sidekick and it becomes my phone? Or do I have to specifically go to a... show more Ok. so I have a T-Mobile family plan that is from 2006-2008. I have a RAZER, but I want to buy the 50 dollar sidekick. How can I do this without having to renew my plan, or change my phone number? Is it possible to just put in my SIM card in any sidekick and it becomes my phone? Or do I have to specifically go to a T-Mobile store and renew everything? THX.
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