Just make sure it will use your sim card and you will be fine. If it doesn't have the sim card you will probably have a problems getting it activated. I thought about switching to T-mobile from Sprint but wanted to keep my phone and since it didn't have that sim they wanted me to get a new phone.
Good thing with T-mobile is that if you get a new phone, all you have to do is take your sim card out and put it in your new phone and will work without having to go to them or call them to get it transfered.