KOMA-Script: change font of sectioning headings to serif

Use \setkomafont or \addtokomafont. See section 3.6 of the KOMA-Script manual for details.

\documentclass{scrbook}

% Variant A: Default plus roman (i.e., still bold)
\addtokomafont{disposition}{\rmfamily}

% Variant B: Ecactly as in the text body (i.e., not bold)
% \setkomafont{disposition}{\normalfont}

\begin{document}

\chapter{foo}

\section{bar}

Some text.

\end{document}

\addtokomafont{disposition}{\rmfamily}

As was suggested here, there is also an option that can be set that also modifies the size of the headings slightly to match that of the standard classes.

\documentclass[headings=standardclasses]{scrartcl}