If AT&T left its downtown St. Louis tower today, the vacancy rate of downtown’s spiffiest office space would more than double to nearly 29 percent. AT&T said last month it will move most of its workers out of the 44-story building. The company says no one will be laid off but that by 2015 employees will relocate to other AT&T buildings downtown or in St. Louis County. A new report from commercial real estate broker CBRE said the anticipated loss of AT&T is partly offset by what appears to be Laclede Group’s decision to move its headquarters to a structure built for General American Life that has been empty a decade. CBRE notes that a Laclede move would simply move jobs from one downtown building to another but would fill a long-empty structure.