Modify locally \tikzset

As suggested, you can use tikzpicture separately for each beamer slide.

The style is passed as an option to the tikzpicture.

Here is the code.

\documentclass{beamer}

\usepackage{tikz}
\usetikzlibrary{mindmap,trees}
\begin{document}
\pagestyle{empty}
\begin{frame}
\begin{tikzpicture}[scale=0.7,transform shape]
  \path[mindmap,concept color=black,text=white]
    node[concept] {Computer Science}
    [clockwise from=0]
    child[concept color=green!50!black] {
      node[concept] {practical}
      [clockwise from=90]
      child { node[concept] {algorithms} }
      child { node[concept] {data structures} }
      child { node[concept] {pro\-gramming languages} }
      child { node[concept] {software engineer\-ing} }
    }
    child[concept color=blue] {
      node[concept] {applied}
      [clockwise from=-30]
      child { node[concept] {databases} }
      child { node[concept] {WWW} }
    }
    child[concept color=red] { node[concept] {technical} }
    child[concept color=orange] { node[concept] {theoretical} };
\end{tikzpicture}
\end{frame}
\begin{frame}
\begin{tikzpicture}[concept/.append style={fill={none}},scale=0.7,transform shape]
  \path[mindmap,concept color=black,text=black]
    node[concept] {Computer Science}
    [clockwise from=0]
    child[concept color=green!50!black] {
      node[concept] {practical}
      [clockwise from=90]
      child { node[concept] {algorithms} }
      child { node[concept] {data structures} }
      child { node[concept] {pro\-gramming languages} }
      child { node[concept] {software engineer\-ing} }
    }
    child[concept color=blue] {
      node[concept] {applied}
      [clockwise from=-30]
      child { node[concept] {databases} }
      child { node[concept] {WWW} }
    }
    child[concept color=red] { node[concept] {technical} }
    child[concept color=orange] { node[concept] {theoretical} };
\end{tikzpicture}
\end{frame}
\end{document} 

The beamer slides would look like this:

\tikzset is local if used properly, that's the main point of putting the style definitions in the options of a tikzpicture or at least in a group, see e.g. this answer which contains Till Tantau's thoughts.

This pot here is just a small addendum to say that there exists the overlay-beamer-styles library which allows you to avoid drawing the mindmap twice. If you do not want to fill the nodes on the second slide, just say

alt=<2>{concept/.append style={fill={none}}}

and so on. So if you change something in your mindmap you only need to change it once.

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{mindmap,trees,overlay-beamer-styles}
\begin{document}
\begin{frame}
\begin{tikzpicture}[scale=0.7,transform shape,
alt=<2>{concept/.append style={fill={none}}}]
  \path[mindmap,concept color=black,text=white,alt=<2>{text=black}]
    node[concept] {Computer Science}
    [clockwise from=0]
    child[concept color=green!50!black] {
      node[concept] {practical}
      [clockwise from=90]
      child { node[concept] {algorithms} }
      child { node[concept] {data structures} }
      child { node[concept] {pro\-gramming languages} }
      child { node[concept] {software engineer\-ing} }
    }
    child[concept color=blue] {
      node[concept] {applied}
      [clockwise from=-30]
      child { node[concept] {databases} }
      child { node[concept] {WWW} }
    }
    child[concept color=red] { node[concept] {technical} }
    child[concept color=orange] { node[concept] {theoretical} };
\end{tikzpicture}
\end{frame}
\end{document}