Buying an iPhone 5c but keeping my plan?

I have a plan on T-Mobile that I'm very happy with. I recently decided to get an iPhone. After doing some research, I settled on the iPhone 5c. Apple sells an unlocked version for contract-free T-Mobile for $549 (this ships with a SIM card). My current phone uses a different type of SIM card, so I would need to have my number/plan transferred to the new one.

Could I just walk into any T-Mobile store (or even just call customer support) and have them change my plan over to the new phone? Or how does that work? I'm not buying my phone from T-Mobile, as the stores near me have a significantly higher price on the same contract-free phone.

Yes, you can always keep your same plan and change phones as long as you had a smartphone plan before.

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