It's official: General Motors Corp. moves its world headquarters from the venerable GM Building (GM types simply call it "The Building) in midtown Detroit to the Renaissance Center downtown, overlooking the Detroit River. GM purchased the "Ren Cen" complex - a 73-story cylindrical hotel surrounded by four 39-story cylindrical office towers - in May for a paltry $72 million. Chairman John F. (Jack) Smith Jr. and other officers relocated on Veterans Day to the 38th and 39th floors of Tower 100 after quickly nixing one proposal to take the 13th and 14th floors.