I have a T Mobile iPhone 4 that I'm having proximity sensor issues with. Can I take my logic board and install it in a AT&T iPhone 4 that I have laying around?
I haven't personally attempted to (never dealt with a T-Mobile iPhone) however since both are GSM based I believe it should be possible and fairly straight forward.
iFixit has a great guide on replacing the iPhone 4 logic board. I would follow that guide, tearing down both iPhones at the same time, looking for any differences. If you come across a difference with the antenna connections, battery connection, etc then I would stop and use your best judgement on whether or not to continue.
An alternative option, if the logic boards are too different to swap, would be to remove and install the AT&T iPhone's display assembly onto your T-Mobile iPhone. This part is less likely to be different than the logic board.
Hope that helps, good luck!