Toggling a class in JS
Use toggle methods

Presuming that you are just toggling those two classes, you could just use classList.toggle() with those two class names:

function toggle_expandable_icon(elem) {
    elem.classList.toggle('fa-plus-circle');
    elem.classList.toggle('fa-minus-circle');
}


And it appears jQuery is used, so the .toggleClass() method could be used on a jQuery collection (i.e. remove the [0] in toggle_expandable_icon($(this).find('i')[0]);):

function toggle_expandable_icon(collection) {
    collection.toggleClass('fa-plus-circle').toggleClass('fa-minus-circle');
}


Those class names could also be added together in a single toggleClass() call:

function toggle_expandable_icon(collection) {
    collection.toggleClass('fa-plus-circle fa-minus-circle');
}


Demo

Without knowing the HTML structure it was challenging to know exactly how the toggles operated, but it wasn't very difficult to generate something.



$('.expandable_header').click(function() {
    toggle_expandable_icon($(this).find('i'));
    $(this).nextUntil('tr.expandable_header').slideToggle("slow");
});

function toggle_expandable_icon(collection) {
    collection.toggleClass('fa-plus-circle fa-minus-circle');
}
&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"&gt;&lt;/script&gt;
&lt;link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.8.2/css/all.css" rel="stylesheet" /&gt;
&lt;table&gt;
  &lt;tr class="expandable_header"&gt;
    &lt;td&gt;
      &lt;i class="fa fa-plus-circle"&gt;&lt;/i&gt;
    &lt;/td&gt;
  &lt;/tr&gt;
  &lt;tr style="display: none;" height="250px"&gt;
    &lt;td&gt;                                                           
                                                                                                                               &lt;a href="https://codereview.meta.stackexchange.com/a/1511/120114" target="_blank"&gt; Zombies exist! &lt;/a&gt;
    &lt;/td&gt;
  &lt;/tr&gt;
&lt;/table&gt;