To modify the definition of a command we use \renewcommand, but how about modify a beamer template. For example if we change itemize item like this:

\defbeamertemplate{itemize item}{triangle} {\scriptsize\raise1.25pt\hbox{\donotcoloroutermaths$\blacktriangleleft$}} 

We get the error:

! LaTeX Error: Command \beamer@@tmpop@itemize item@triangle already defined.

This is an example of that:

\documentclass{beamer} \defbeamertemplate{itemize item}{triangle} {\scriptsize\raise1.25pt\hbox{\donotcoloroutermaths$\blacktriangleleft$}} \begin{document} \begin{frame} \setbeamertemplate{itemize item}[ball] \begin{itemize} \item text \item text \end{itemize} \setbeamertemplate{itemize item}[tiangle] \begin{itemize} \item text \item text \end{itemize} \end{frame} \end{document}