Renewing the \@ptsize command

Question

Why the command below don't change the size of the font? I have copied it from book.cls

\documentclass{book}

\makeatletter
\renewcommand*\@ptsize{2}
\makeatother

\newlength{\hallo}
\settowidth{\hallo}{m}

\begin{document}

Hallo World

\the\hallo

\end{document}

---

Could I have some problem with this way? For example if I put the \@newglobalfontsize{} command after some packgaes in the preamble?

\documentclass{book}

\makeatletter
\DeclareRobustCommand*{\@newglobalfontsize}[1]
{
    \let\small\relax
    \let\footnotesize\relax
    \let\scriptsize\relax
    \let\tiny\relax
    \let\large\relax
    \let\Large\relax
    \let\LARGE\relax
    \let\huge\relax
    \let\Huge\relax
    \makeatletter
    \input{bk#1.clo}
    \makeatother
}
\makeatother

\makeatletter
\@newglobalfontsize{10}
\makeatother

\newlength{\hallo}
\settowidth{\hallo}{m}

\begin{document}

Hallo World

\the\hallo

\end{document}

UPDATE

To avoid all problems this is the best way for me:

\documentclass{scrreprt}

\makeatletter

\DeclareRobustCommand*{\@newglobalfontsize}[1]{\changefontsizes[\dimexpr#1*1.2\relax]{#1}}

\@newglobalfontsize{8pt}

\makeatother


\begin{document}

test

\end{document}

Answer 1

By the time you modify \@ptsize the document class already set up the font sizes, so the change is not realized. Using

\documentclass[12pt]{book}

is the preferred way. More specifically, the document class options are set up (with some defaults) and then processed, followed by the appropriate font setup (reading bk1\@ptsize.clo, for example) inside the document class (book.cls and the like).

From book.cls (with comments added):

128: ...
129: \ExecuteOptions{letterpaper,10pt,twoside,onecolumn,final,openright}% Defaults
130: \ProcessOptions% Process options passed by user + defaults
131: \input{bk1\@ptsize.clo}% Read/setup font-specific information
132: ...