Beamer - make itemize (and space occupied) disappear

One can simply combine both itemizations in a single one:

\documentclass{beamer}

\begin{document}

\begin{frame}   

\begin{itemize}
    \item<only@1> item on first slide
    \item<only@1> more item on first slide
    \item<only@2> item on second slide
    \item<only@2> more item on second slide 
\end{itemize}

\includegraphics[width=5cm]{example-image-duck}

\end{frame}

\end{document}

Use \alt or \temporal:

\alt<n>{at n}{not at n} for two alternatives.

\temporal<n>{before}{at n}{after} for three alternatives.

More information at http://saikat.guha.cc/ref/beamer_guide.pdf