redefine theorem to be tcolorbox

amsthm and tcbtheorem cannot be used interchangeably, that means that your best shot to replace the one with the other is to use your editor's search and replace functionality (or sed or other nifty tricks in vim or emacs).

One thing you can do without changing our document, is using the skinning mechanism with tcolorboxenvironment:

documentclass[11pt,a4paper,ngerman]{scrreprt}
usepackage[utf8]{inputenc}
usepackage{babel}
usepackage{amsthm}
usepackage{thmtools}

newtheoremstyle{break}{0pt}{0pt}{normalfont}{0pt}{bfseries}{}{newline}{thmname{#1} thmnumber{#2} quad thmnote{normalfont#3}}
declaretheorem[style=break,name=Theorem]{theorem}

%% a colored Theorem style
usepackage[most]{tcolorbox}
tcbuselibrary{theorems}
usepackage{cleveref}

newtcbtheorem[
number within=chapter,
crefname={Theorem}{Theoreme},
]{theoremTCB}{My Theorem}%
{colback=green!5,colframe=green!35!black,fonttitle=bfseries}{th}

tcolorboxenvironment{theorem}{ 
    breakable,
    colback=green!5,
    colframe={green!35!black},
}

%% want to replace 'theorem' to be the colored 'theoremTCB'
%% what to do with label  ?
%renewenvironment{theorem}[args][default]{begdef}{enddef}

begin{document}
chapter{Intro}
begin{theorem}[Optional title]
label{myfirstTheorem}
    blabla
end{theorem}
this is the ref: -- cref{myfirstTheorem} --. 

The Theorem shell be redefined to look like:

begin{theoremTCB}{Hey}{marker}
    hello
end{theoremTCB}
this is the ref: -- cref{th:marker} --. 

end{document}