Upgrade Device Software on a Unlocked 8100

I have a T-Mobile Pearl 8100, and I unlocked it because I was moving to the Dominican Republic, and the phone has been working well since I unlocked, but now I want to upgrade the Software on the device, because I'm running the 4.2.0.64 version. But I've been trying to upgrade it from T-Mobile website but they ask me for my T-Mobile cell phone number which I don't have because I'm using the Carrier "Claro" in the Dominican Republic. what OS should I download? I was thinking about the AT&T one... and the other question is If I upgrade my OS will my phone remain unlocked. Thanks a lot!!