Overwriting an External Index Style File from filecontents Environment

Question

I'm using the filecontents environment to make a small index format inside a tex input file as follows:

\begin{filecontents}{format.ist}
    headings_flag 1
    heading_prefix "{\\bfseries\\hfil"
    heading_suffix "\\hfil}\\nopagebreak\n"
    item_0 "\n \\item"
    delim_0 " \\dotfill"
\end{filecontents}
\begin{document}

Now if I'm for example to change the headings_flag to (-1) for lowercase, this new change does not get applied in the external format.ist file. Is there a way to immediately apply those changes made in the tex file to the style file at each run?

I have used the filecontents environment as proposed in another question but it didn't work.

Thanks, in advance.