I switched mine (from iPhone to "smartphone") yesterday. But it was only because I had to move my iPhone to a transferred line to be bundled with my account for my wife. The CSR simply asked what I was replacing my phone with, of course had no idea what a Nexus One was, and then gave me the whole "we cannot be responsible for your device and we're never going to help you because it's not an AT&T phone" disclaimer. (Which is kind of funny, because they don't support the iPhone, either. They just make you go to Apple.) But they do seem to be rather picky in verifying the phone to data plan matchup.
I assume it would work (as suggested above), but I wonder if they see a large increase in non-BIS/BlackBerry data if they would question you.