\documentclass[a4paper,11pt,DIV9,BCOR0mm,twoside,headinclude]{scrartcl}
\usepackage{scrpage2, calc}
\newlength{\smallmarginparwidth}% zur Speicherung der normalen Breite

% Die folgende Anweisung muss nach eventuellen \typearea-Aufrufen
% wiederholt werden!
\setlength{\smallmarginparwidth}{\marginparwidth}

\newcommand*{\addtomarginparwidth}{.5\smallmarginparwidth}
\makeatletter
\newcommand*{\widemarginpar}{% Umschalten auf breite Marginalien
  \clearpage
  \ifdim\marginparwidth=\smallmarginparwidth
    \addtolength{\textwidth}{-\addtomarginparwidth}%
    \addtolength{\marginparwidth}{\addtomarginparwidth}%
    \addtolength{\evensidemargin}{\addtomarginparwidth}% for 'twoside'
    \@typearea@end
  \fi
  %die Position der Kopfzeile justieren:
  \chead{\makebox[\widthof{\chtext}]{\hspace*{\addtomarginparwidth}\makebox[0cm]{\chtext}}}
  \cehead{\makebox[\widthof{\chtext}]{\makebox[0cm]{\chtext}\hspace*{\addtomarginparwidth}}}
  \ohead{\makebox[0cm][l]{\hspace*{\addtomarginparwidth}\makebox[0cm][r]{\ohtext}}}
  \lehead{\makebox[0cm][r]{\makebox[0cm][l]{\ohtext}\hspace*{\addtomarginparwidth}}}
  \pagestyle{scrheadings}
}
\newcommand*{\smallmarginpar}{% Umschalten auf schmale Marginalien
  \clearpage
  \ifdim\marginparwidth=\smallmarginparwidth\else
    \addtolength{\textwidth}{\addtomarginparwidth}%
    \addtolength{\marginparwidth}{-\addtomarginparwidth}%
    \addtolength{\evensidemargin}{-\addtomarginparwidth}% for 'twoside'
    \@typearea@end
  \fi
  \chead{\chtext}
  \ohead{\ohtext}
  \pagestyle{scrheadings}
}
\makeatother


\newcommand{\chtext}{Mitte}     %  \chead; \cehead
\newcommand{\ohtext}{\pagemark} %  \ohead; \lehead
\ihead{Aussen}
\cfoot{}
\ifoot{}
\ofoot{}

\begin{document}

\smallmarginpar
\section{Ohne Marginalien}
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
\clearpage
\section{Ohne Marginalien}
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
\widemarginpar
\section{Mit Marginalien}
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
\marginline{Margin Mar Mar Mar Mar Mar Mar Mar Mar Mar Margin}
Text Text Text Text Text Text Text Text Text Text Text
\clearpage
\section{Mit Marginalien}
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
\marginline{Margin Mar Mar Mar Mar Mar Mar Mar Mar Mar Margin}
Text Text Text Text Text Text Text Text Text Text Text

\smallmarginpar
\section{Ohne Marginalien}
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
\clearpage
\section{Ohne Marginalien}
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text
Text Text Text Text Text Text Text Text Text Text Text

\end{document}
