Sounds basically like what i did when i moved from Virgin Mobile to O2. I don't know, or have had any experience with that phone, however, you can go into a T-Mobile shop and ask for help setting it up to use their network, they're usually happy to help and most of the time i've seen people getting ot done for them free of charge. It's only the changing af a few settings, these are more than likley just the GPRS settings in the phone and of course and WAP or messaging settings (for MMS)
The settings can be found by a quick google search, such as the listings here (which i have used before for the O2 settings) http://www.filesaveas.com/gprs.html
I don't know however how to get to the settings in the first place
however the T-Mobile staff should.
A google search on that may help too