Refactoring the legacy code which is using '§' char in the code
I don't see you using std::, surely you didn't put using namespace std somewhere in your code right?

I find your parameter names a bit confusing. stringToReplace is actually the replacement. The string you want to replace is stringToFind. Maybe use something like replaceWith or just replacement for the second parameter instead.

There is a type mismatch which your compiler should have caught (no warnings enabled?). The return value of find is size_t.

You can also pass the first argument as a reference and the other two as const reference.