Macro for promoting sections to chapters

One option:

\documentclass{book}

\let\subparagraph\paragraph
\let\paragraph\subsubsection
\let\subsubsection\subsection
\let\subsection\section
\let\section\chapter

\begin{document}

\tableofcontents
\section{Test section to chapter}
\subsection{Test subsection to section}
\subsubsection{Test subsubsection to subsection}
\paragraph{Test paragraph to subsubsection}
\subparagraph{Test subparagraph to paragraph}

\end{document}

An image of the document with the \let lines commented out:

An image of the same document with the \let lines active:

The ToC:

and the first page of the body: