Check if a node field has changed upon save
Clive is correct.
$node->original does give access to the original node values. I was able to accomplish what I needed like so

function my_module_node_presave($node) {
    $type = $node-&gt;getType();
    if($type == 'issue'){
            if(isset($node-&gt;field_pdf-&gt;entity) &amp;&amp; !isset($node-&gt;original-&gt;field_pdf-&gt;entity) || empty($node-&gt;original-&gt;field_pdf-&gt;entity) || $node-&gt;field_pdf-&gt;entity-&gt;id() != $node-&gt;original-&gt;field_pdf-&gt;entity-&gt;id()){
                $node-&gt;field_indexed-&gt;value = false;
            } else {
                $node-&gt;field_indexed-&gt;value = true;
            }
    }