Changing Related Post Selection From Tags to Categories

Hey Everyone.

I am hoping this is the right place to ask this question.

I am using a new WordPress theme and it has native related post coding in it. The problem is the code uses Post Tags as the selection value instead of Post Category. I was wondering if anyone could help me with this strip of code. I want to change it so the theme gets Posts in the same Category, instead of Posts with the same Tags.

* Display related posts.
 *
 * @return void
 */
function stag_related_posts() {
	global $post;
	if( stag_rcp_user_has_no_access() )
		return;
	if( ! stag_theme_mod( 'post_settings', 'show_related_posts' ) )
		return;
	$tags = wp_get_post_tags( $post->ID );
	if ( count( $tags ) ) {
		$tags_ids = array();
		foreach($tags as $individual_tag) {
			$tag_ids[] = $individual_tag->term_id;
		}
		$query = new WP_Query( array(
			'tag__in'          => $tags_ids,
			'post__not_in'     => array( $post->ID ),
			'posts_per_page'   => absint( stag_theme_mod( 'post_settings', 'related_posts_count' ) ),
			'caller_get_posts' => 1
		) );
		// Add filter to attach 'post-grid' class
		add_filter( 'stag_showing_related_posts', '__return_true' );
		if( $query->have_posts() ) :
			echo '<section class="related-posts" data-layout="2-2-2-2">';
			while( $query->have_posts() ) : $query->the_post();
				get_template_part( 'content', get_post_format() );
			endwhile;
			echo '</section>';
		endif;
	}
	wp_reset_query();
	remove_all_filters( 'stag_showing_related_posts' );
}

I’ve tried simply replaces the words Tags with Categories, but it doesn’t seem to do much.

Any help would be greatly, greatly, appreciated.