I had an iPhone on AT&T and really hated it. I had a 1000 minute plan + data plan. I did not change my plan, pulled out my SIM, placed in my Nokia E71. Everything worked and i kept my plan to term on the "iPhone contract".
If you don't pull your data plan and keep it, and if the cost is about the same as the T-Mobile data plan for Android, just switching the SIM should work.
Alternatively, if you have a local T-Mo rep that is good, just go in one day and talk to him in person. My local guy is great and as long as you will keep your minutes and switch over to an Android dataplan, I think they would do it for you without charging you.