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It may depend on being prepaid vs postpaid. If prepaid, you'll need to buy the card. If postpaid, it's likely a tossup and will depend on the rep you get.
It may also be that the T-Mobile store will want to charge you for a new SIM but will cut the SIM for you at no charge.
When my wife got her Moto X and needed a different SIM size, we walked into the local T-Mobile store (Authorized Retailer rather than corporate, in case that matters) and walked out with her phone activated on a new SIM five minutes later without being charged.
It may vary from store to store, but my experience has been that they will swap out SIMs for free.