There's a certain peculiarity that seems to exist among Chicagoans: the absolute resistance to change*. Anyone how thinks it would be funny to point out Obama, a Chicagoan, ran a campaign based entirely on the concept of "change" can just shut up now, because that's not my point. Sears Tower will only be called Willis Tower by the local media, because they have to. Non-media locals a.k.a. everyone else, including my suburban self, will continue to call it Sears Tower because that's what it always has been (plus, if you couldn't guess, Sears is a Chicagoland company). Change is fine for the President and Chester A. Bum (another fine Chicago suburbanite), but not I. Hell, die-hard (no pun) fans of the White Sox still refer to the field as Comiskey, even though U.S. Cellular bought the naming rights six years ago, and when New York based retailer Macy's bought Chicago based retailer Marshall Field's (and changed the store name to Macy's as they had done so many times before), sales plummeted. Changes to decades old Wrigley Field almost always meet resistance.
The point of that long, rambling block of text? Willis can call the tower whatever the hell they want, but good luck convincing the locals to call it that.
*Aside from the several locals posting here who have no problem with the name change. I have no idea where they came from. And if the absolute resistance to change exists elsewhere, I don't know about it.