Anyone know if we can use the Rogers Amber rom on ATT Lumia 920? That way we're not switching from RM-820 to RM-821 and can possibly revert back to ATT when the update is released.
Unfortunately, no. Once you flash any other rom including the Rogers rom, there's no going back to AT&T.
The whole point of flashing - at least for me - is to unbrand the phone, remove all the AT&T stuff and get the cool apps like Data Sense. You do lose the ability to tether but if you rarely or never use it, it's not a huge deal. If your main concern is to just get the GDR2/Amber update because the whole world is getting it and not you, then just hold out a little longer if you can.