KOMA-Script: How to style the title of a chapter

Second update

Here is a new and in my opinion better suggestion that needs KOMA-Script Version 3.19 or newer. In version 3.19 there is a new command \chapterlineswithprefixformat that could be redefined to use \MakeUppercase for the chapter titles.

\documentclass[english]{scrreprt}[2015/09/29]% needs version 3.19 or newer
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{blindtext}

\KOMAoption{chapterprefix}{true}
\renewcommand*\raggedchapter{\centering}
\RedeclareSectionCommand[beforeskip=0pt,afterskip=8\baselineskip]{chapter}
\setkomafont{chapterprefix}{\normalsize\mdseries}

\renewcommand*{\chapterformat}{%
  \chapappifchapterprefix{\nobreakspace}\thechapter\autodot%
  \IfUsePrefixLine{%
    \par\nobreak\vspace{-\parskip}\vspace{-.6\baselineskip}%
    \rule{0.9\textwidth}{.5pt}%
  }{\enskip}%
}

\renewcommand\chapterlineswithprefixformat[3]{%
  \MakeUppercase{#2#3}
}

\begin{document}
\tableofcontents
\chapter{Chapter One}
\textbf{\KOMAScriptVersion}
\par
\Blindtext
\addchap{Chapter without number}
\Blindtext
\end{document}

Note: If you also want to use \MakeUppercase without the option chapterprefix=true you have to redefine the command \chapterlinesformat:

\makeatletter
\renewcommand{\chapterlinesformat}[3]{%
  \@hangfrom{#2}{\MakeUppercase{#3}}%
}
\makeatother

---

Update

My original answer (see below) works with both MiKTeX2.9 and TL 2015 but not with 2014 (but you can load fixltx2e to get it to work as @Johannes mentioned in a comment). So here is another suggestion.

\documentclass[english]{scrreprt}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{blindtext}

\KOMAoption{chapterprefix}{true}
\renewcommand*\raggedchapter{\centering}
\newif\ifmakeupper
\newcommand*\chaptertitleformat[1]{\ifmakeupper\MakeUppercase{#1}\else#1\fi}
\addtokomafont{chapter}{\makeuppertrue}
\setkomafont{chapterprefix}{\normalsize\mdseries}
\renewcommand*{\chapterformat}{%
    \MakeUppercase{\chapappifchapterprefix{\nobreakspace}}\thechapter\autodot%
    \IfUsePrefixLine{%
      \par\nobreak\vspace{-\parskip}\vspace{-.6\baselineskip}%
      \rule{0.9\textwidth}{.5pt}%
    }{\enskip}%
}
\RedeclareSectionCommand[beforeskip=0pt,afterskip=8\baselineskip]{chapter}

\renewcaptionname{english}{\contentsname}{\chaptertitleformat{Contents}}

\begin{document}
\tableofcontents
\chapter{\chaptertitleformat{Chapter One}}
\textbf{\KOMAScriptVersion}
\par
\Blindtext
\addchap{\chaptertitleformat{Chapter without number}}
\Blindtext
\end{document}

Note, you have to redefine \tablename, \listfigurename, \listtablename, ... so that they use \chaptertitleformat and you have to insert this in all \chapter commands. But as an advantage now there is only a switch in the \addtokomafont command.

---

Original answer (does not work with Version 3.19)

Here is a suggestion but note that \MakeUppercase inside \setkomafont or \addtokomafont can be problematic. In any case \MakeUppercase have to be the last command added to the komafont of a fontelement because it needs an argument.

\documentclass[english]{scrreprt}[2015/02/07]
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{blindtext}

\KOMAoption{chapterprefix}{true}
\renewcommand*\raggedchapter{\centering}
\addtokomafont{chapter}{\MakeUppercase}
\setkomafont{chapterprefix}{\normalsize\mdseries}
\renewcommand*{\chapterformat}{%
    \chapapp~\thechapter%
    \par\nobreak\vspace{-\parskip}\vspace{-.6\baselineskip}%
    \rule{0.9\textwidth}{.5pt}%
}
\RedeclareSectionCommand[beforeskip=0pt,afterskip=8\baselineskip]{chapter}

\begin{document}
\tableofcontents
\blinddocument
\end{document}

A bit alternative approach:

\documentclass{scrreprt}
\let\raggedchapter\centering

\setkomafont{chapterprefix}{\normalsize\mdseries}
\KOMAoption{chapterprefix}{true}
\renewcommand{\chapterheadmidvskip}{%
    \par\nobreak\vskip -.1cm {\rule{.9\textwidth}{.5pt}}\par%
}
\RedeclareSectionCommand[beforeskip=0pt,afterskip=8\baselineskip]{chapter}
\usepackage{xpatch}
\tracingpatches
\xpatchcmd{\@@makeschapterhead}{%for the unnumbered
    \usekomafont{chapter}{#1}\par
}{%
    \usekomafont{chapter}{\MakeUppercase{#1}}\par
}{}{}
\xpatchcmd{\@@makechapterhead}{%for numbered
    \usekomafont{chapter}{#1}\par
}{%
    \usekomafont{chapter}{\MakeUppercase{#1}}\par
}{}{}
\renewcommand*{\chapterformat}{%
    \mbox{\MakeUppercase{\chapappifchapterprefix{\nobreakspace}}\thechapter\autodot
    \IfUsePrefixLine{}{\enskip}}%
}

\begin{document}
\tableofcontents
\listoffigures
\chapter*{Bussy Baboon}
\chapter{walzing Wombat}
\end{document}