This should be really straightforward to do - you have a lot of options available with an unlocked GSM phone.
T-Mobile runs off the software technology GSM meaning your device is compatible with other GSM carriers (AT&T, Cricket, etc)
I have created a search results page from our website of all the available carriers that you could bring your unlocked T-Mobile (GSM) device too.
Click this link to Compare BYO Unlocked GSM Plans.
Now from that search results page, I have also created a list of carriers that might be beneficial to you. Each one of these carriers will be compatible with your device making the switch super easy.