documentclass{beamer}
usepackage{tikz}
  begin{document}
  usetikzlibrary{decorations.text, positioning}

begin{frame}{only<1>{AA}
    only<2>{BB}
  }
  resizebox{.85paperwidth}{!}{
    begin{tikzpicture}
      clip<1> (0.0, 0.0) rectangle (10.0, 2);
      clip<2> (2, 2) rectangle (6, 12);

      draw [help lines] (0,0) grid (25,13);
end{tikzpicture}
}
end{frame}
end{document}