As often as you want. Just pay full price for whatever GSM compatible unlocked or T-Mobile branded phone you want they can be new or used. Then take your SIM card out of your current phone and pop it in the new one. That's it, unless your phone needs some special settings input but that isn't T-Mobiles problem. Although they'd probably help you.
Heck you could have twenty phones sitting around and switch the SIM card between them everyday. If you were so inclined.