documentclass[11pt]{scrbook}

usepackage{titlesec}
newcommand{sectionbreak}{clearpage}%this is global effect

begin{document}
chapter{introduction}
%Need Command Here To turn OFF Starting Each
%Section On New Page After The First Section
  section{A}
   blabla
  section{B}
   blabla

chapter{Another chapter}
%Need Command Here To turn ON Starting Each
%Section On New Page After The First Section
  section{A}
   blabla
  section{B}
   blabla

end{document}