You can just buy any device you want to use from T-Mobile and then move the Sim card from your current device over. If you need a different Sim due to the size, you can get another one and have them update the card to be used with your current plan. You will not be required to change plans. You will just need to pay full price for the device. And buying the phone from T-Mobile, should have a Sim card in the box still I believe.
Manage your T-Mobile account