I have a Tmobile postpaid SIM in my phone. It is not turned into a Tmobile phone. AGain, what you saw when you put the SIM in was the screen telling you the phone was setting up the Tmobile APNs. That's just it's way of saying it's configuring the network settings to work with Tmobile. It doesn't change anything about the software though to make it a Tmobile phone. As far as Tmobile is concerned, you still just have some unlocked phone on their network. You can use it, but that's the entirety of the support they provide for it. All software updates are done by the manufacturer.