sawyourwolfjunk is correct, you replace calls to expand_link() with print_expand_link(). So any time you find "->expand_link" in your code, replace it with "->print_expand_link".

The second part of my explanation was to split up strings IF changing expand_link to print_expand_link causes you to have problems with the spacing characters (the characters that show up before/after the expand link in your html). To do this, you just put each string on a separate line/expression. In your case:

" " + $c->print_expand_link();

becomes

" ";
$c->print_expand_link();

and

"(" + $comment->print_expand_link() + ") ";

becomes

"(";
$comment->print_expand_link();
") ";

but you only need to do that after you've replaced "->expand_link" with "->print_expand_link", if you have noticed it's botching your spacer characters. Mine did for some reason but this may not be the case for all.