Walgreens will move about 1,800 employees into the renovated old main post office in Chicago, Mayor Rahm Emanuel will announce on Monday.Walgreens will take over a 200,000-sq.-ft. office space when it is fully open next year, a statement from the mayor's office said on Sunday. This will be the largest number of corporate employees Walgreens has ever had in Chicago.Renovations to the behemoth building began after the city reached an agreement with the iconic building's new owner last year.