\documentclass{article}

\usepackage{xparse}

\let\oldsection\section
\RenewDocumentCommand{\section}{s o m}{%
  \IfBooleanTF{#1}
    {\oldsection*{\MakeUppercase{#3}}}% \section*
    {\IfValueTF{#2}
       {\oldsection[\MakeUppercase{#2}]{\MakeUppercase{#3}}}% \section[.]{..}
       {\oldsection{\MakeUppercase{#3}}}% \section{..}
    }%
}

\begin{document}

\tableofcontents

\section{A section}
\section[Another section]{A section}

\end{document}