documentclass{beamer}
usepackage{amsmath}
usepackage{tikz-cd}

begin{document}
begin{frame}[fragile, t]{My slides}
only<1>{
begin{equation*}
begin{tikzcd}
a
end{tikzcd}
end{equation*}}
only<2>{
begin{equation*}
begin{tikzcd}
a  arrow{r} &  b
end{tikzcd}
end{equation*}}
end{frame}
end{document}