August Mack Environmental said today it will move its corporate headquarters from Castleton to downtown’s Lexington at Meridian. The Indianapolis-based environmental consulting company plans to take 16,200 square feet in the former home of VanAusdall & Farrar Inc., joining Brown Mackie College, which is taking 25,500 square feet. Brokers say such a migration, away from the suburbs, is rare and should bode well for the downtown office market. The Lexington, which is undergoing a $10-million renovation by Shiel Sexton, offers some of the perks of suburban offices, including parking on site. The building now is 100 percent leased, with the project a couple of months away from completion. Shiel also plans to build new office space surrounding the historic building. The space was marketed by Colliers Turley Martin Tucker.