Replace pagenumber for one page in scrbook

I have a two sided scrbook where I want to replace one single page number with a special char, while the rest of the pages should remain numbered with arabic numbers. How can I achieve this?

So far I have tried it with fancyhdr and with redefining pagemark as you can see in the MWEs, but both did not bring the intended result.

With fancyhdr the pagenumber resp. rfoot is not put in the same place as for the normal scrbook style since I use mpinclude=true:

documentclass[twoside,mpinclude=true]{scrbook}

usepackage[showframe]{geometry}
usepackage{fancyhdr}
usepackage{lipsum}

title{Book}

begin{document}
maketitle

pagestyle{fancy}
renewcommand{headrulewidth}{0pt}
lfoot[{:-)}]{}
rfoot[]{{:-)}}
cfoot{}
section*{Acknowledgements}
I want to thank everyone.

clearpage
pagestyle{plain}
lipsum

end{document}

When I redefined pagemark it is in the right place but I do not know, how to reset it to the previous definition.

documentclass[twoside,mpinclude=true]{scrbook}

usepackage[showframe]{geometry}
usepackage{lipsum}

title{Book}

begin{document}
maketitle

lipsum[3]

clearpage

renewcommand{pagemark}{:-)}
section*{Acknowledgements}
I want to thank everyone.

clearpage
pagestyle{plain}
lipsum

end{document}

documentclass[twoside,mpinclude=true]{scrbook}

usepackage[showframe]{geometry}
usepackage{lipsum}

title{Book}

begin{document}
maketitle

lipsum[3]

clearpage

renewcommand{pagemark}{:-)}
section*{Acknowledgements}
I want to thank everyone.

clearpage
renewcommandpagemark{usekomafont{pagenumber}thepage}
lipsum

end{document}