It looks to me AT&T only took over the accounts in your area but not the towers because they would not be giving out FREE Phones other wise.. Towers are built with CDMA or UMTS..
No, a TOWER is simply a structure. It doesn't care what's on it. AT&T bought everything here. The Alltel CDMA gear is useless (though will be maintained for roaming purposes at Sprint's demands for three years). They're adding UMTS equipment to the Alltel towers, and also massively increasing backhaul capacity to support HSPA speeds.