I remember a buddy having to upgrade his tdma phone to a (gsm?) phone so it's certainly possible.
Found this old article using google that says they're reallocating 2G bandwith to improve 3G. Have no idea if they're a valid source of info though.
Fewer Bars in More Places: AT&T Network Upgrades Degrade Service for 2G Phones - OFB.biz: Open for Business
I know I'm finally getting a good 3G signal at work so they did something. It was switching at lot to edge when the iPhone 3G/3GS was released.