Reusing the same tex files in different base files

Question

I have a number of .tex files containing sections which are \input{} into a large document. I'd now like to create an "excerpt" document that includes only a subset of these sections. The problem is, the hierarchy level should be different in the excerpt than in the main document. That is, in the main document the concerned section may be a \subsection, but in the excerpt it must be a \section (i.e. everything moves up a level). Is there a way to handle this hierarchy level correctly in different files using \input{}?

Answer 1

Since in comments you imply that you want to change the relative levels rather than just move a branch up or down the hierarchy, I'd use custom commands in the input files.

If qqqq.tex looks like

\sectA{zzzz}
zz zz zz
\sectB{thing}
xx xx xx
\sectB{more things}
ww ww ww

Then one document can go

\newcommand\sectA{\section}
\newcommand\sectB{\section}
\input{qqq}

with everything at the same level, and another can go

\newcommand\sectA{\subsection}
\newcommand\sectB{\subsubsection}
\input{qqq}

with A headings a level down and B headings two levels down