Change Beamer frame background between slides in same itemize

I think what you want is not possible. Take a look at How does beamer build the frame? and will see that background canvas and background are constructed before anything else and the frame contents is the last piece, after titles and sidebars. Therefore even trying to supply a false background with TiKZ overlay and remember picture this background will cover any previous layer.

Next code shows a manual solution built playing with \visible and \invisible. You'll need to build as much as frames as items and if your slides are numbered, you'll need to find how to adjust them.

\documentclass{beamer}

\usepackage{mwe}

\begin{document}
\usebackgroundtemplate{\includegraphics[width=\paperwidth,height=\paperheight]{example-image-a}}
\begin{frame}{Promotion}
\begin{itemize}
\item\visible<1>{Identifying the target audience}
\item\invisible<1>{Understanding the target audience}
\item \invisible<1>{Identifying partners}
\end{itemize}
Some text
\end{frame}

\usebackgroundtemplate{\includegraphics[width=\paperwidth,height=\paperheight]{example-image-b}}
\begin{frame}{Promotion}
\begin{itemize}
\item\visible<1>{Identifying the target audience}
\item\visible<1>{Understanding the target audience}
\item \invisible<1>{Identifying partners}
\end{itemize}
Some text
\end{frame}

\usebackgroundtemplate{\includegraphics[width=\paperwidth,height=\paperheight]{example-image-c}}
\begin{frame}{Promotion}
\begin{itemize}
\item\visible<1>{Identifying the target audience}
\item\visible<1>{Understanding the target audience}
\item\visible<1>{Identifying partners}
\end{itemize}
Some text
\end{frame}

\end{document}