The changes at GM that put Mary Barra in the CEO spot probably won't have an immediate impact on the company's $3 billion U.S. ad budget. While Barra is careful and in that sense adhere's to GM cultural aversion to and sometimes obsession with risk, she's also a crusader against the "hidebound" culture at the automaker. But its unlikely that there will be a Joel Ewanick-style shakeup (when Ewanick headed GM marketing there was a sense of constant, almost frenetic flux in strategy and agency lineups.