As in changing the phone name in the build.prop to a nexus? I'm not really sure what that would do. I'll look up the model number and report back what happens when I change it.
As for flashing this, I doubt it will cause any more problems than the root kernel and bloat removal will. Unless AT&T has some spyware buried into the phone, they shouldn't know, nor should they care.
This flash literally just tosses in the permissions XML and adds the control program from the other phones to the system apps. EVERYTHING else was already in the ROM, including the Nfc.apk and libraries. Kinda annoying, but AT&T is wanting it locked out on phones until they're proprietary NFC payment system is released.
Personally, I wanted to play with launch tags, so I figured out how to unlock it, and then made a CWM flashable that does all the heavy lifting I figured out by tearing through the Rodgers and Telus ROMs.
To alcohol! The cause of, and solution to, all of life's problems!