\let\newCommand\oldCommand
\renewcommand\oldCommand}{\doThat\newCommand\doThis}
now you can use \newCommand and \oldCommand
Example: an article-layout starts a new section always on the same page. Starting on a new page is possible with
\let\mySection\section
\renewcommand\section{\newpage\mySection}
in this definition \mySection is always the same as the old \section-command.
A \renewcommand inside a new environment looks like
\newenvironment{booglewog}{%
\renewcommand{\bog}[2]{##1 ##2}%
}{%
}
Otherwise the two Parameters do not belong to \bog.
Especially the definitions for the parts/chapters/sections/... have a star definition, too. If this one should be redefined too, then do it in this way:
Possible Syntax for \foo (\part, \chapter, \section, ...): \foo*{...}
\foo[...]{...}
\foo{...}
The star is catched by \@ifstar:
\newcommand*{\oldfoo}{}% check, if \oldfoo is free
\let\oldfoo\foo
\renewcommand*{\foo}{%
\@ifstar{%
\starfoo
}{%
\@dblarg\nostarfoo
}%
}
\newcommand*{\starfoo}[1]{%
\dosomethingbefore{#1}%
\oldfoo*{#1}%
\dosomethingafter{#1}%
}
\newcommand{\nostarfoo}{}
\def\nostarfoo[#1]#2{%
% #1: toc entry
% #2: main entry
\dosomethingbefore{#2}%
\oldfoo[{#1}]{#2}%
\dosomethingafter{#2}%
}