After following the instructions on this link, I was able to make it work using Bootstrap4-CSS-fontAwesome without any JS or jQuery.

<a class="accordion-toggle collapsed" data-toggle="collapse" href="#anyId">
  click
</a>

<div id="anyId" class="collapse">
    Hi
</div>

<!-- CDNs for font-awesome, bootstrap, JQuery -->
<link href="//netdna.bootstrapcdn.com/font-awesome/3.2.1/css/font-awesome.css" rel="stylesheet">

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" rel="stylesheet">

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>

<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/js/bootstrap.min.js"></script>

.css

.accordion-toggle:after {
  font-family: 'FontAwesome';/* essential to enable caret symbol*/
  content: "\f0d7";/* adjust as needed, taken from font-awesome.css */
  color: grey;
}
.accordion-toggle.collapsed:after {
  /* symbol for "collapsed" panels */
  content: "\f0da";  /* adjust as needed, taken from font-awesome.css */
}

Special thanks to @Satheesh Kumar

If you are working with SCSS, there are specific built-in mixins for handling this situation. These mixins can be found in the following location:

~bootstrap/scss/mixins/_caret.scss

These mixins include caret-down, caret-up, caret-right, caret-left, and caret($direction).

Implementing these mixins can be as simple as:

.panel-heading .accordion-toggle {
    @include caret(right);

    &.collapsed {
        @include caret(left);
    }
}

Alternatively, if your HTML includes the aria-expanded attribute (which is recommended)

.panel-heading .accordion-toggle {
    &[aria-expanded="false"] {
        @include caret(left);
    }

    &[aria-expanded="true"] {
        @include caret(right);
    }
}       

My approach to achieving this was by utilizing only CSS for bootstrap 4.2 and fontawesome. I incorporated aria labels and implemented transitions for the icon through a span class.

CSS:

#accordionExample .btn[aria-expanded=false] span:before {
  font-family: "Font Awesome 5 Free";
  font-weight: 900;
  content: "\f107";
  float: right;
 transition: all .5s;
}

#accordionExample .btn[aria-expanded=true] span:before {
  font-family: "Font Awesome 5 Free";
  font-weight: 900;
  content: "\f107";
  float: right;
  transition: all .5s;
  -webkit-transform: rotate(180deg);
    -moz-transform: rotate(180deg);
    transform: rotate(180deg);
}

HTML:

<div class="container my-5 mw-600">
  <div class="accordion" id="accordionExample">
  <div class="card">
    <div class="card-header" id="headingOne">

        <a class="btn btn-link d-flex" data-toggle="collapse" data-target="#collapseOne" aria-expanded="false">
          Collapsible Group Item #1
          <span class="ml-auto"></span></a>

    </div>

    <div id="collapseOne" class="collapse" aria-labelledby="headingOne" data-parent="#accordionExample">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
  <div class="card">
    <div class="card-header" id="headingTwo">

        <a class=" btn btn-link collapsed d-flex align-items-center" data-toggle="collapse" data-target="#collapseTwo" aria-expanded="false">
          Collapsible Group Item #2
          <span class="ml-auto"></span>
        </a>

    </div>
    <div id="collapseTwo" class="collapse" aria-labelledby="headingTwo" data-parent="#accordionExample">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
  <div class="card">
    <div class="card-header" id="headingThree">

        <a class="btn btn-link collapsed d-flex" data-toggle="collapse" data-target="#collapseThree" aria-expanded="false">
          Collapsible Group Item #3<span class="ml-auto"></span>
        </a>

    </div>
    <div id="collapseThree" class="collapse" aria-labelledby="headingThree" data-parent="#accordionExample">
      <div class="card-body">
        Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
      </div>
    </div>
  </div>
</div>
</div>

Demo: https://codepen.io/anon/pen/XoLgbY

After testing out various solutions, I decided to implement Vinni's approach as it appeared to be the most straightforward. However, I encountered an issue where the icon displayed as a box in Chrome and Firefox, while Safari and Chrome mobile showed it correctly. Upon further investigation, I discovered that the newest version of Font Awesome requires the addition of font-weight: 900; when utilizing solid icons in custom CSS classes. This was necessary for icons like the caret (f0d7) to display properly. In addition, I made sure to include font-family: 'Font Awesome\ 5 Free'; instead of just opting for font-family: 'FontAwesome'; The updated CSS code now looks like this:

.accordion-toggle:after {
font-family: 'Font Awesome\ 5 Free';
font-weight: 900;
content: "\f0d7";
color: grey;
}
.accordion-toggle.collapsed:after {
content: "\f0da";
}

Another option to consider is using CSS exclusively.

.navbar-nav .dropdown.open .caret{
    -webkit-transform: rotate(180deg);
    -moz-transform: rotate(180deg);
    transform: rotate(180deg);
}